I agree with all who have responded to my comment about "not exposing Cassel" in that Vikings game in London. That game speaks more to our own failings and not to Cassel's abilities. I agree that Cassel has always been overrated due to his success with New England that one season. He should be a #2 somewhere, but that's about all he'll ever be from now on in my opinion.
I still think (personal feelings and bad PR aside) we are much better off with Vick than Cassel. Vick doesn't have to be a world beater to spread out a defense with the tools on this offense, he just needs defenses to believe he is capable of burning them deep if they stack the box. Vick can do that. Cassel can't.
I'm also counting on Vick's maturity and acceptance of his role now. Plus, I think Ben respects Vick, and it always helps if the #1 and #2 QBs establish a good working relationship and ease at communicating. If Vick sees something from the sidelines and points it out to Ben, I believe Ben will hear it better from a QB he respects. Ben's relationship with Batch was always a plus, and he likes having a #2 that he can speak to.