It's not so much about Usher per se, but rather the underlying concept of the movie (cute white girls fawning over black guy, black guy beating up white guys.) Of course Usher is better than, say, that disgusting creep, 50 Cent.
I don't think he's a very good singer though. A lot of those black R&B singers have whiny, high-pitched voices. Yech. (Of course, so do many of the white "rock" singers. Yech again.) What happened to good singing? It's a vanishing art form.