hmm. Katy, Beyonce, Bruno (except for the dance off portion), Coldplay, and BEP (though they should have...

) didn't lip. It's not really the norm there. Especially if you consider the old rock acts that dominated for those several years like Prince, Bruce, and The Who. I think only Madonna lipped. lol
The problem with Rihanna performing a 12 minute continuous set, despite all her hits is that... she'd make everyone regret liking them. At most she could perform for 5 minutes. And that's pushing it.