Can it ever be justified as a long term change?
A lot of people complain about Red Hulk joining the Avengers. I'd say General Ross (by becoming a Hulk and joining the team) is basically doing what he thinks the Hulk should be doing. Protecting others as a "big gun" secret weapon. His military background makes him a strong team player as well. Although he was extreme, he was never malicious. Therefore he should stay a good guy.
Although he had some under handed intentions, I'd say joining an organization like thev Future foundation sort of fits with Doom's style. Proactively educating young minds in how to make a difference in the world... "With Science!" How can Doom not resist taking part? Though, with his history with Reed, I don't see him staying for long. Doom should stay neutral (rather than good or bad).
Lost his mutant powers (went through a lot to get them back). And the Mutant population was severely reduced. It's in Magneto's character to want to work together with the remaining Mutants and maybe rethink his views of humans. In particular, Mags is generally described as being "Malcolm X" to Xavier's "MLK jr", Malcolm X changed his view of non-blacks after a pilgrimage and Magneto has basically done the same. Let him stay good.
What do you think? Any other cases to be discussed?