You can make a very strong case that for the majority of users buying gear from the AH will put you further ahead as opposed to levelling up the blacksmith.
However, this relies on the prices in the auction house remaining low. This should be a safe, long term assumption as the supply should outpace the demand assuming people resell items they purchase.
The best gear that the blacksmith produces is certainly being hoarded by those who have invested the time and effort to generate it or will be offered for sale at an enormous price.
In the short term, yes, you can do better in the AH under the current economy.
In the long term, dependant very strongly on your luck in crafting equipment with the stats you are hoping for, the blacksmith may become your better source of equipment as the best Legendary items will almost certainly be absurdly priced.
Crafting equipment has always entailed some risk. In Diablo 2, the best players would hoard perfect rubies to try to create "Blood" gear (items with strong life leech). If your build calls for such a piece then it may be the small advantage you need to start succeeding in Inferno.
Many won't want to do this but will benefit from the few who are doing it. If they're looking for dex gloves and you want int, their failures are your opportunity to buy.
Spirit Walk is a skill that fits into a category that I like to call "Panic Button". These skills can have other uses, but they're generally skills that are very good at getting you out of a bad place.
The Witch Doctor has a few other ones.
Horrify
This will pretty immediately clear the space around you of enemies. It will give you a few seconds to catch your breath and set up for the battle to continue. If you rune it for 100% bonus armor for 8 seconds, that will give you an additional boost for 4 seconds as the battle continues. Alternatively, you could rune it for move speed to help you get out of there as fast as possible.
Hex
If you are being chased by only one enemy, Hex will immediately chicken that enemy and allow you to catch your breath and get some bonus damage in. You can heal up and space yourself appropriately while he's a chicken so you can be ready when he comes back into his normal form. This particular tactic was the only way I was able to beat Izual on Hell difficulty solo. For some reason, Izual can be chickened, although I don't think any other bosses can. It's also particularly effective against rare monsters.
Grasp of the Dead
You need to either rune this for 80% slow or reduced cooldown for it to be particularly effective as a defensive ability. Both of these are solid choices, but the 80% slow against any enemy without the Fast affix is as good as just stopping them outright. This is very effective as a pre-emptive panic button. If you see a huge horde of enemies running at you, retreat to a choke point and cast Grasp of the Dead to prevent them from coming near you.
Fetish Army
The Fetish Ambush rune will get you out of a bind quite effectively. You spawn 6 fetishes, each dealing 250% weapon damage in an area when they spawn. This will usually kill everything in your immediate vicinity, and then you will have an army of meat shields to help you position yourself properly afterwards.
Spirit Vessel
This passive will save you from death once, but it really works best in conjunction with Spirit Walk. If you're using Horrify or Soul Harvest, however, this can definitely be a viable choice even without Spirit Walk.
Ultimately, however, Spirit Walk accomplishes multiple things at once, which none of these other spells do. Out of all of these options, none have the sheer utility of Spirit Walk. I recommend, if you insist on not using Spirit Walk, using at least two or more of these other abilities. I actually use Spirit Walk, Grasp of the Dead, and Hex most of the time for maximum survivability, and I still occasionally get cornered and have nothing to do but maximize my DPS before I die. Spirit walk is really, really good.
Best Answer
Each difficulty level has it's own crafting materials. For example, in Nightmare Mode, Subtle Essence is replaced by Shimmering Essence, and Fallen Teeth is replaced by Lizard Eyes.
Once you start needing drops from a higher difficulty to train your smith in higher levels, you can know that you won't be able to craft anything learned at those higher levels without materials from those higher difficulties.
So, in short, No, there's no real benefit to doing this. You can also browse all of the crafting recipes, sorted by Smithing level over at Blizzards Game Guide site. As you can see, Level 5 Crafted items learned via training require level 30 or above, at which point, you should be in Nightmare mode. If you're learning these recipes while you're still in Normal Mode Act II, you're either severely overleveled, or you're already at a point where you can craft things long before you're able to use them.