He quit and therefore should pay the consequences for that, but at the same time, the band could've been nicer about it considering it was their last show, and Brendan could've at least responded. But either way, the band's not in the wrong for denying him the right to play. It just wasn't an unreasonable request.
I kinda feel like since he quit, his request to be a part of the whole thing was a bit much. Definitely think he should be a part of it, but hey, he quit the band. I think the band's suggestion of playing on some of the hits was a very fair compromise, though I could understand him being upset.
I think the band was generous in offering him to play their hits considering he was no longer a part of Valencia. You can't just quit a band then expect to be treated like a full member again even if it was for their last show.
In addition to him quitting, they also had a guy come in and learn all their songs, so to tell that guy that he was being replaced would have been a major slap in the face. However, he went on to say that the band offered to have him play the encore, which I think would have been the best solution. Its a tough situation, and the way JD explained it, I don't think anyone was going to come out looking good in it.
I pretty much agree with what everyone else has said.
On one hand, I completely understand him asking to be a part of the show since Valencia was at one point such a huge part of his life.
By that same token, in a previous post on his health-related blog, he had said he quit the band essentially because he no longer felt invested in the music and lifestyle that being in Valencia led to. So I also completely see where the other guys were coming from in being hesitant to say yes straight off.