Duskblade 13/Crusader 1/Jade Phoenix Mage 6 seems to me to be much superior to Duskblade 20. You hit the really big duskblade class feature and retain your highest-level spells.
The issue is timing. The best levels to take Crusader are 5th, 9th, or 13th. Coincidentally, however, 5th, 9th, and 13th are also the levels where it is most painful to have a single level of crusader rather than being a single-classed duskblade, as those are the levels at which you get new levels of spells (and full-attack arcane channeling, in the case of 13th). You have two competing goals: get the next spell level (and full-attack arcane channeling) as soon as possible, while having maneuvers for as long as possible, since they are useful.
Therefore, I recommend taking the crusader level at 6th, 10th, or 14th; that is, I’d take the next spell level first, rather than the maneuvers. Here’s a breakdown of each option:
Duskblade 5/Crusader 1
You can take 2nd-level maneuvers with your first five maneuvers. This means, primarily, that you will have mountain hammer for the longest possible time, which is awesome. Other than that, though, you’ll probably still take a number of 1st-level maneuvers.
Duskblade 9/Crusader 1
You can take 3rd-level maneuvers and stances with your initial set; you can skip crusader’s strike for revitalizing strike, you can still take mountain hammer, and the amazing white raven tactics is available to you. Thicket of blades opens up as an interesting stance option; martial spirit is pretty small at these levels, and thicket of blades greatly improves your presence on the battlefield.
Duskblade 13/Crusader 1
Clearly if you haven’t taken crusader by this point, you should. Duskblade has little to offer, so you lose almost nothing at this point. The disadvantage of waiting this long is that you haven’t had maneuvers at all until this point. The advantage, on top of duskblade features ASAP, is divine surge, an excellent offensive maneuver.
The Awkward Bit
OK, so now that we’ve covered how to do it, there’s one other thing to consider: you cannot use arcane channeling and a martial strike at the same time. Arcane channeling requires a standard action attack or a full-attack, so the attack(s) you get from a strike don’t count. This makes strikes massively less useful to you than they would be to others. A boost or counter heavy selection of maneuvers solves this issue; you could do pretty well with defensive rebuke, shield block, and white raven tactics. But missing out on fantastic options like mountain hammer, tactical strike, the various healing strikes, and divine surge is a disappointment.
It also means you don’t have especially great mobility. You have invested 13 levels in duskblade to allow you to full-attack with spells, so you really do want to be making full-attacks. Crusader doesn’t help with that. Swordsage or warblade would help a little, since Tiger Claw is pretty good at that (sudden leap, arguably pouncing charge), but cleric for Travel Devotion or barbarian for Lion Spirit Totem (Pounce) might be better. Those options don’t lead into a great prestige class like jade phoenix mage, but they still might be better options.
The Other Thing
Finally, item-based healing is generally sufficient in 3.5. In-combat healing is rarely an optimal strategy, barring emergencies; the crusader is far better at this than other classes, but you shouldn’t usually need it. The aura you describe all-but-eliminates the worst emergencies, too. Wands and healing belts are quite sufficient for out-of-combat healing. As Jeor Mattan mentioned in comments, the real draw of actually having a cleric is the various options he or she would provide for removing status conditions, since items have HP-healing pretty much covered (and until you get heal, clerics’ HP-healing spells are pretty poor), and crusaders don’t cover that at all.
This is another situation where a level of cleric may be more useful to you: it would allow you to use wands of any cleric spells you like, with no Use Magic Device check. That will cover a lot your needs.
First, ask your DM if you can qualify for abjurant champion with an exotic weapon; you don’t need proficiency in any martial weapons to take Exotic Weapon Proficiency, and there are some better options for exotic weapons.
If exotic weapons are not allowed: Longbow
There’s not a whole lot of point to getting any martial weapon; any situation where attacking with a weapon is a better option than spellcasting, the fight is either already won and you’re conserving resources, or else you would be better off getting out of there.
Therefore, you might as well take something you can trivially and safely plink at enemies with: the longbow is that. A crossbow of any stripe is awful without Rapid Reload, which you don’t want to spend a feat on. Either really wants Precise Shot, which is not an awful feat for a spellcaster, but still not really worth it. Particularly when it requires Point-blank Shot and you can just buy a rod of magical precision to cover your spells’ precision needs.
Attach a least crystal of return from Magic Item Compendium to it, because most of the time you’d probably rather have a wand or rod in hand; the few cases where you want it, you might as well be able to use it the turn you want it, and 300 gp is a paltry sum for Quick Draw.
If exotic weapons are allowed...
This gets more interesting. There are a number of exotic weapons with useful properties.
As general notes:
Good Exotic Weapons discussion
Haberdash the Masked – A build for a master of masks, which has an option, the gladiator mask, to gain proficiency with all weapons. Includes a thorough list of exotic weapons worth considering.
Dungeon Master’s Guide II has the Feycraft weapon template. For 1500 gp, a one-handed melee weapon can be made eligible for Weapon Finesse, or a light melee weapon can be made to use Dexterity even if you don’t actually have Weapon Finesse. Considering your stats, Feycraft should be applied to any melee weapon that qualifies.
- While you’re at the fey smith’s, consider picking up a feycraft mithral chain shirt and stuffing it full of thistledown padding: abjurant champion will make a +4 AC, −0 ACF, 0% ASF armor kind of redundant, but you can throw armor properties on it, which is supremely useful, and furthermore you can be caught without your luminous armor or have it dispelled.
A least crystal of return is still a good idea. I seriously tend to put one of these on every weapon or weapon-like item I ever buy, because free actions are so very nice.
...a braid blade gives free attacks (Dungeon vol. 120)
When you full-attack, and are wielding a braid blade, you get to make a free attack with it. This is a no-caveats, no-penalties, no-feats, stacks-with-everything extra attack à la haste or Rapid Shot. You’ll want something else for your other attacks, most likely; a quarterstaff or dagger is fine.
...a gnomish calculus lets you hurl alchemical weapons (Arms & Equipment Guide)
Also, it’s one of the most ridiculous (and ridiculously-cool) weapons ever printed, and has by far the best name of any weapon. The gnomish calculus allows you to hurl alchemical weapons like tanglefoot bags much farther than you otherwise could.
This is mostly pointless; alchemical weapons mostly stop being useful at like, level 3. But it’s still cool.
Gets massively better (but still mediocre) if your DM rules that, as ammunition for the calculus, your alchemical items get any weapon properties applied to the calculus. This is arguably RAW (the alchemical items are referred to as ammunition, and no exception to the usual rules for projectile weapons imparting their enhancements on their ammunition is made), but it’s an unusual enough situation that I would ask.
Finally, it’s described as a fancy sling. Gnomish or not, halflings are better with these than anyone else.
...a greatbow outdamages a longbow (Complete Warrior)
For the dead-simple upgrade to the martial suggestion.
...halfling skiprocks are allow you to attack twice as much (Races of the Wild)
If you hit someone with a skiprock, you get to make a free attack against anyone within 5 ft. of the first target at a −2 penalty. Requires tight enemy clustering, but can potentially double your number of attacks.
Plus, they count as ammunition, and therefore cost 1/50 to magically enhance compared to other weapons. So you can get dirt-cheap weapon properties, particularly stuff that don’t actually involve attacking like eager, warning, or defending. Honestly, you don’t need proficiency for this, but your DM might feel a little bit better about the cheesiness if you are proficient.
...harpoons let you apply some mundane battlefield control (Frostburn)
The guy you impale with this moves at half speed, and cannot run or charge. For a lot of enemies, that’s tantamount to saying they have no offense; you can do a lot worse with a weapon. They can remove it as a full-round action – but it deals its damage again, and they just wasted their turn. If an enemy actually does that, you should thank them for it.
Also conveniently a thrown weapon, which is takes advantage of your being a halfling.
...a long staff can make you impossible to flank (Complete Adventurer)
You have to use the Total Defense option, or take Combat Expertise, but still, you get the option of eliminating flanking. If you’re hurting, just need to survive a round, and have a pair of rogues on you, this is effectively a huge boost to AC plus totally wrecking their damage output. It’s niche as hell, but man is it awesome when it does work.
Then again, you should almost-definitely be getting heavy fortitude on that feycraft armor or on a mithral buckler sooner rather than later. (Soulfire from Book of Exalted Deeds is your other priority, if you were wondering; having a +1 soulfire feycraft mithral chain shirt and a +1 heavy fortitude mithral buckler is my usual goal for armor.)
...a rope dart (meteor hammer) has the best reach (Dragon vol. 319)
This thing is ridiculous: 15-ft. continuous reach, à la the whip, but none of that nonsense about not threatening or failing against armor. Two-handed, but it can be finessed by default; ask your DM if you can apply feycraft to it and if so, whether it works as it does on light weapons. RAW, neither is true.
There are actually two weapons presented in Dragon vol. 319: the rope dart, which is described in detail, and then the meteor hammer, which is exactly like a rope dart except that it deals bludgeoning damage. Proficiency in one counts for the other.
...or a whip-dagger (Arms & Equipment Guide, Dragon vol. 353, Dungeon vol. 134)
If you don’t get to apply feycraft to the meteor hammer, you can ask after the whip-dagger instead: same reach, one-handed-but-finesseable weapon instead. No threatening with that one, but at least you can attack armored foes with it.
...or kusari-gama (Dungeon Master’s Guide)
If that doesn’t fly, the kusari-gama from the Dungeon Master’s Guide is a sure thing: it’s a light weapon that is otherwise a lower-damage spiked chain (in complete defiance of reality, I should add). Less reach on it, though.
...a razor net annoys enemies (Dragon Compendium)
This is basically a net that deals 1d6 damage, cuz why not. Nets can pretty much shut someone down; that’s highly useful.
Feel free to grab the regular net if Dragon Compendium isn’t available; the 1d6 damage is minuscule anyway.
Actually, consider grabbing (razor) nets even if you don’t take proficiency in them. They’re a touch attack anyway; even at −4 you can reliably hit that on some enemies. Plus, thrown weapon, so you’re already starting at +1.
...a Xen’drik boomerang qualifies for Boomerang Daze (Races of Eberron)
I know, it’s another feat. It’s also absolutely ridiculous. You get to push a hard Fortitude save versus daze on every single attack. This is so good that it will be better than casting a spell fairly often. Nothing else on this list is even close to claiming that. Daze is a status condition that very-nearly-no-one is immune to. Favor of the martyr is a 4th-level paladin spell that provides immunity to daze for 1 round/level – and it is just about the only thing that does that, so shenanigans to get it onto non-paladin spell lists are common. Anyone who hasn’t engaged in shenanigans (and isn’t a high-level paladin) is otherwise vulnerable to this.
If you want double-down on this, and spend another feat, Boomerang Ricochet can allow you to daze two people per attack.
Also, it’s thrown, so your halfling bonus applies.
Best Answer
No, there is no good balance reason
The designers may have imagined there was; it’s conceivable they considered the option of Weapon Finesse as your 1st-level feat (or Weapon Focus, Exotic Weapon Proficiency, Quick Draw, and so on) to be important advantages of full-BAB classes. I tend to believe it was more of an “imagery” thing, though – full-BAB classes were the “weapon masters” and so weapon-related feats tended to get this requirement. Not because balance demanded this, but rather because their image of what each class was “supposed” to be like. But either option is pure speculation on my part, nothing more.
The theory:
Regardless of what they were or were not thinking when they wrote the feat, the BAB +1 requirement on Weapon Finesse is not only unnecessary (just as it is not for Weapon Focus, Exotic Weapon Proficiency, or Quick Draw), but is also exceptionally problematic. That’s because, unlike the other feats I mention (with the possible exception of Quick Draw), Weapon Finesse is a build-defining feat – someone with Weapon Finesse is going to have low Strength and high Dexterity, and is going to be interested in melee combat. Thus, they need Weapon Finesse to function correctly.
Worse, this description, of a high-Dexterity, low-Strength melee combatant, does not describe most full-BAB classes – you can make a Dex-based fighter, certainly, and even a Dex-based barbarian with the right supplemental material, but these are not the norms for those classes. Fighters get proficiency with heavy armor and shields as a major class feature; you cannot use those on a Dex-based build. Paladins are worse – same proficiencies, but far fewer feats. And rangers are worst of all – their ability to ignore the Dexterity requirements of their Combat Style feats is a major class feature, one of the only ones that is both relatively unique and relatively useful for that class.
And then you have medium-BAB classes that, unlike the full-BAB classes, are often perfectly described as high-Dex, low-Str melee combatants. Monks and rogues wear light armor if they wear armor at all, have ¾ BAB, and find themselves in melee. Rogues in particular see a lot of advantage from high Dexterity: requirements for Two-Weapon Fighting feats, associated score for critical skills. These are characters who want to build around Weapon Finesse.
Which sucks if you are level 1, and have BAB +0. You are forced to sit through two whole levels of hideously poor accuracy, just so you can make the math work by taking Weapon Finesse at level 3. This doesn’t add anything to the game, it just makes those classes suffer needlessly.
The practice:
The BAB +1 requirement on Weapon Finesse (and most other feats, honestly) is waived in the overwhelming majority of games I play in (the only games where it isn’t, that I can recall, are those in which no one wanted it and therefore didn’t bother asking the DM), and every single game I DM. In fact, most of the people I play with forget that it has that requirement. And our games are better for it. It allows us to play at low levels without being forced to suffer through unnecessary glitches like those caused by Weapon Finesse’s requirement.
I strongly recommend every DM to waive that requirement. I’d also recommend doing so on other, similar feats (certainly each of the ones I listed earlier). To not do so, I think, is a mistake. A mistake that the authors of Player’s Handbook made, true, so an understandable mistake, but a mistake nonetheless.
In fact, I would echo Thales Sarczuk’s suggestion, and strongly consider just making Weapon Finesse a free option.