Slash himself played in a MJ song or do you forget?
But Slash isn't a musician, he's a hired gun.
Slash may be a "hired gun", but he's still a great musician. And quite frankly, a way better one than Kurt Cobain.
Quite frankly, I think anyone getting pissy about this is just being whiny. Oh waaaaah! I don't want to be a celebrity, so I'll publicly sell my music! Yeah, fucking stupid if you ask me. Rock Band and Guitar Hero both are about deserving and recognizable rock musicians. To bitch about a musician that you like being on one of them is ridiculous and you all know it is.