If Mangini seriously thought he would have just as much creative input as the others, joining a band of 25 years at the time, then that was really naive of him. You don't just walk on in like you own the place after getting the gig and say, "Okay guys, I'm thinking we should do this, this and that for this album. Thoughts?" It's surely a privilege you gain slowly, over time. The same was true of JR when he joined DT. His creative input slowly became more and more welcome with each album, and you can hear it. If Mangini is actually frustrated and unhappy (not saying he is), then he just needs to chill out and be patient.
It's not naive at all. He expected to be in the writing process and not what he's done as a hired gun before. Come into a polished song an add his florishes.
First off, that's exactly what he got to do on the self-titled album...he got to come up with his own drum parts, and even had some input in the songwriting process. On ADToE, JP had a rough outline of programmed drums for the tunes, but Mangini still got to elaborate on them quite a bit. With The Astonishing, all of the music was written, and while JP did have a lot of say in the matter, Mangini got to come up with all kinds of drum parts - as long as they followed the flow of the composition, and were approved by JP. If anyone thinks that JP sat hunched over a computer for hours programming all these nuanced drum parts for The Astonishing, they're flat-out wrong. I specifically asked him at the meet and greet to what extent did he develop the drum parts, and he said he had a basic idea of the timing; whether a section would be half-time, whether it would be very upbeat and busy, or a laid back groove, etc. He didn't have very specific drum parts in mind - Mangini got to come up with a lot more than what people have been giving him credit for. In the end JP had final say on parts, yes, but he didn't sit down at a computer and program everything and hand it to Mangini and say "here, just play this". It was more along the lines of "that's great, try this instead" and "that was awesome, can you add more this, less that".
Secondly, I think you missed the point of what I was saying earlier. When he
first joined the band, it would've been unrealistic of him to think he had equal creative input as the others, being brand new in a band that has been established for over twenty years. He's done great though, being the drummer and being able to be there and play what is requested of him at times, but also being able to infuse his style into the music as well. Again, I'm not saying he is unhappy based on that interview - I'm saying that if he actually is, he needs to realize that those privileges come with time. Or, if he feels really strongly about it, he should talk to the guys.