Short answer: Yes
Longer Answer: Yes, you can use them as long as you have the necessary skill for the item (i.e. you can't wear the plate chest with your druid). The EXP-Bonus will work (tested this myself with my cloth wearing druid ;) ).
However, the stats on said items may not be exactly what you are looking for, but I guess this is really obvious. :)
If we look at low level Rogue abilities such as: Sinister Strike, Backstab, Ambush, etc. We see a consistent trend of a percentage of Weapon Damage. Damage for these abilities is calculated:
base weapon damage + constant * (attack power)/14
In the case of daggers this constant is 1.7 and other one handed weapons 2.4. This is known as AP normalization. Additionally, only the main hand weapon is used in these calculations. As a result it is ideal to use a slower main hand weapon until you have sufficient attack power to influence your choice.
In the case of Rogues it is common to use Daggers as certain abilities (such as Backstab) require them. At latter levels you will gain other abilities which influence this selection (Combat Swords, Mutilate, Hemorrhage, etc), but at lower levels, slow Main Hand daggers is a preferred choice. A longer discussion of Main Hand weapon choices can be found here.
As a subtlety Rogue your "bread and butter" ability is Hemorrhage which does 110% MH damage (159.5% if its a Dagger as Daggers tend to be faster). I would suggest Initiative + Improve Ambush, making your combat look as follows:
- Ambush (or cheap shot)
- Gouge (later you'll use Kidney shot later in the order).
- Backstab (or Hemorrhage)
- Finishing Move
- Hemo -> Repeat till Gouge/Cheapshot come back up or you have the Combo Points to Finishing move
Garrote can be a good opener if the target stays alive for the full duration. As most mobs won't while leveling, Ambush is usually the better choice.
As a Subtly Rogue you also trade on Dodge, Ap, Crit and Armor pen (the last of which you won't get in bulk till much later). Keep in mind that Agility gives you 3/4, but is not as cost effective as straight AP due to Deadliness.
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Best Answer
Assassination spec
Two daggers. You need them for Mutilate and Assassin's Resolve from level 10. Go get your Scarlet Kris heirlooms.
Combat spec
Not daggers. Some abilities reference one strike (Sinister Strike, Revealing Strike, Killing Spree, Mastery). That strike should be big. Dagger's one strike is small. Other abilities are normalized so speed doesn't help (Blade Flurry, Combat Potentcy). Use Thrash Blade or Mass of McGowen heirlooms.
Subtlety spec
Daggers allow Backstabs and bigger Ambushes. However you might not be using Backstab, Hemorrhage gives faster combo point generation. Faster combo points means more finishers. I find that in current dungeon situations, fights are very fast and backs available - Dagger shines here. While solo'ing, backs are not as available, or against dungeon bosses, fights are longer and finishers matter more.
Level 63 subtlety rogue vs level 60-63 Zangarmarsh wildlife:
Knowing this, I would choose Dagger for Subtlety. Hemorrhage does the same damage either way. Ambush does 25% more damage with dagger. If you aren't ambushing and backstabbing, weapon choice doesn't matter.