I think you are thinking of Ambush, which requires stealth, not backstab. Backstab does not require stealth...
Those are the only two moves which require a dagger...
You are looking at the big # damage, but you have to consider damage over time (or DPS - Damage per Second)...
Using a sword, you may get higher damage per hit, but you'll get fewer hits in a given time-period. With daggers, which are generally faster, you'll get slightly less damaging hits, but more of them in that time-period.
This can be very useful for spell interruption...
If you are using a sword with less DPS than a dagger, you're not only hurting your Damage over the course of the fight, but also missing out on a very useful move (Backstab). Gouge and Backstab are a wonderfully effective combo.
Now, all that said, play what you like, play what fits your style, just keep in mind, DPS is what matters most. WoW is accomodating to actually figure out the DPS and list it on the weapon stats, there's a reason for that...
Edit: Took me too long to post, some of my babble has already been clarified Edited, Tue Dec 7 09:09:41 2004 by Malyr