Rossi is done. He's no where near the pace of Lorenzo. MotoGP these days favors a really compact body type to get into those ridiculous lean angles, Rossi is just too tall.
Marquez is really young, the pressure on him is massive. Lorenzo has got nothing to lose by going all out in Valencia. Either way, motoGP2013 has been one of the best seasons ever, certainly more exciting than Snormula Yawn.
I'm concerned that Yamaha will let Rossi run around on their top bike for a long time, because he's good for marketing, but it's hurting them overall. Lorenzo could have used a good teammate to slow up the Repsol duo more than a few times this season, only to have him get passed with no troubles.
I think you could tell this weekend that he was feeling some pressure. He wasn't his normal giddy self. He has been able to say all season that it wasn't his job to win the championship, and there is no pressure to do so. Now there is, and it's getting close.
He only needs a 4th place, but it's a fine line to walk by trying to coast to the line. When riders get out of their groove, bad things start happening. Or, he could end up in a fight with the normal Rossi/Bautista/Bradl/Crutchlow group, and that's the sort of thing that he doesn't want. I'm assuming he will go for it - Marquez only knows one speed, and that's full speed. The only way Lorenzo will win the championship is if Marquez crashes. Which could happen.