Katie has little or no voice, which I will attest to, but at least she changes things up. She went from terribly bad to just bad, but is improving her voice and stage presence. While she may never get into the top six, she will try harder than anybody there to work it. She's going in the right direction trying to sing "young" music and appear her age.
Tim has looks, and that's all, and the reason he is still here. He gets the biggest cheers each and every week, but if you listen, it's teen and twenty something girls who do the cheering. He has as much rock and roll street cred as Donny Osmond or Leif Garrett.

He is so so boring, I would far more enjoy watching a Katie concert. Watching a Tim concert would put me to sleep after the third song. Tim should be an actor or male model, but stay away from singing.
The only good thing about Tim becoming a rock star is it would be great for bra and pantie sales, as tons of female fans would be tossing their undergarments up on the stage. It's insane how his picture has gone up on blogs and how girls have just gone crazy over this guy. Remember when he sang his boring song but there were the rabid female fans trying to touch him? It shows part of this show is not about singing, but about sex appeal. Now if Tim had Adam Lambert's voice, then I can see the screaming female fans, but Tim has those screaming fans nonetheless.
I have the awful feeling that Tim will outlast Katie, Andrew, Didi, Aaron, and Casey.
Crystal, Mike, Lee, and Sibohan are safe from Tim's loyal female following and will advance to the top 4. I think this could be Lee's contest to lose.
Here's my prediction. In the finals, the judges use their one save and will "save" Crystal, thus making another finals, to boost the sagging ratings of this season. Lee vs. Crystal, twice. It's the only way to bring Idol back to it's former top status.