so, I did the snowglobe quest but, without thinking, I just went in, talked to the dryads killed the dragon and left. later I learned that I was supposed to leave and go back to talk to the dwarves so they could fix the amulet of ages. now here's my question: is the amulet used for anything other than to help you kill the guardian dragon?

The fixed amulet gives you some extra protection against cold attacks and such. I'd look up the exact bonusses but don't have NWN here, so I can't.
Repairing the amulet will doubtlessly get you some extra XP, but IIRC it is too specialised to be of very much use (you'll have equallly good all-round things by the time you find it).