No, it wouldn't be unbalanced. Dervish is already severely limited by the fact that you have to move between each attack and the duration (until you get tireless dance, it fatigues you after) is based on skill ranks, something that's impossible to pump up beyond the norm. As opposed to Barbarian Rage, where you can min-max your Con score for longer duration.
Especially if you allow the later 3E books that actually gave melee nice things and made skirmishing viable (Tome of Battle, PHB2 [Bounding Assault feat line], and C.Adventurer [Scout class] in particular), Dervish, with its massive feat pre-reqs and limitations, actualyl becomes pretty freaking weak.
Also, check out Tome of Magic sometime. A level 5 Binder (level 3 if he purchases a very good feat most binders will want anyway) can bind Paimon. This gives several benefits, including but not limited to: free access to the Whirlwind Attack feat, an untyped +4 to dexterity, and an ability VERY similar to dervish dance. It allows you as a full round action to move up to your speed and take one attack against any foe you move past. You have to wait 5 rounds to use it again (or 4 if you get a feat, and there's another easy method to be able to do it 2 rounds in a row 1/day that has no real cost at all). There is no restriction on what weapon you can use at all. Yeah, you can't spam it repeatedly, but you also get it at lower level and MUCH lower investment cost.
So, based on all that...no, I don't think it'd be overpowered to just let Dervish attack with any weapon during a dance.
EDIT: I forgot about that! Thanks, Kitcik. Travel Devotion feat and Lion Spirit Totem (both C.Champion) ALSO completely obsolete the entire prestige class. Poor dervish, I like it so much...
