Perhaps some of the groups aren't that popular and porcupine is not aware of them. I never heard of Amy Winehouse until a few weeks ago.

There is some good rap and there is some idiotic rap and there is some that is a little of both. "Gold Digger" and "Baby Got Back" are both brilliant - and both extremely sexist. Some words from "Gold Digger" could get one's ass handed to one if spoken in a certain contexts. It's still genius, though. OTOH, "hollaback girl" is musically brilliant, but culturally just stupid.

Rap music is singled out because it is often (not always, but often) far more abrasive and more pervasive than other stuff.
Possibly the reason this is frowned on is the perception that kids actually try to emulate the behavior they hear on these songs.