I swear that I've seen it done, if not well, at least not terribly, before, but I can't remember where. (I guess I should point out that the Bedlam's Bard series is actually one of those stories, but I can't remember how Eric Banyon is dragged into it originally, and in the later series he's familiar with all of it. So those books might count.)

Huh, actually, I guess Harry Potter sort of fits into that category, come to think of it, though I wouldn't classify HP as urban fantasy.

"Only you can save the world" stories, while occasionally done well, usually make me twitch around the edges.