Entertainment. I'm not saying characters shouldn't change, I'm just saying it's not a necessity. Wouldn't there be something satisfying about a movie (any movie, not IM2 or anything) where a truly awful person is thrown bewildering obstacles and manages to overcome them while still retaining awfulness by the end to the frustration of others? It sounds almost pessimistic (which it's not), but there's something just interesting and funny about that.
There are some prominent examples in comedies, but I guess they would be pointless to list because they're comedies. One superhero movie you could make the argument for is TIH. I know people say character development lacked a little in that movie, but character development is just developing the idea of the character, not the changes in them. Anyways, the only slight change you could say for that movie was Banner accepting that the Hulk could be used for good. But who's to say he never thought so before? The point of the story is to watch a man overcome the outstanding challenges presented to him by others or fate and see how he does it, observe his relationships with others, and enjoy the ride.
I definitely agree with you that one of the most important aspects in a movie is progression. If nothing goes anywhere there's not a good chance the movie will be good (though who's to say nobody can pull that off...). I'm just saying that doesn't necessarily umbrella the characters themselves. If every movie were like that though, most probably would suck. In fact, maybe that's why the idea appeals to me so much, it's just hardly done. It's the reason I love the very polarizing movie Observe and Report. That movie does pretty much everything you're not supposed to do in a movie, including a reformed/progressing protagonist. It changed the way I thought about how they say things "have to be done" and opened up my head a little more to other possibilities.
But it really all depends on the type of movie you're making. Now specifically for IM2, do I think Tony Stark should go through some changes? Yes. I think he should learn to be a little more responsible and learn to cut back on his reckless behavior (I haven't seen the movie yet). But like Downey said about the first IM, he shouldn't go through this absolute reform and become Mr. Goody-Two-Shoes all of a sudden. That just doesn't happen to people, "When someone used to be a schmuck and they're not anymore, hopefully they still have a sense of humor." So like I said earlier, I do think Stark should maintain his egomania and ******* tendencies, just have it checked by others now and then (ie. Pepper, Rhodey, Fury, Cap etc.).
And that's all I gotta say about that.