What, the fuck, are you talking about? Two examples of why this is an insane statement... Listen to Zep, then Metallica, then Matchbox 20, and tell me how rock music is so "limiting".
Example 2? Listen to any Marilyn Manson album, then listen to another. That's just ONE guy and he's managed to make 7 or so albums that all sound completely different from eachother.
But rap beats, now there's the path to artistic nirvana.