Tali came in handy when I was playing as a Soldier, since I didn't have any decryption skills, she could hack computers and decrypt locks for me (she's basically the equivalent of a rogue). Having her in the squad also improves the Mako's repair rate. In combat I give her a shotgun and try to keep her away from the frontlines, she has some handy skills like overheating enemy weapons and disabling shields.
I think in ME2 there might be a romance possibility since on the official forums it seemed to be in huge demand (and lack of it was a common complaint about the first game). Some really weird folks out there. Although if she removes her bulb-helmet and turns out to be pretty hot, I'm game.
