Edit: also, you're right about checking the ego. I've been trying to do that. It's just hard to look at a film like Stealth or Ecks vs Sever and not think "I could write circles around the guy who wrote that." But I know that there are a lot more things that go into making a crappy movie than the talent/ability of the screenwriter.
Thinking like this will only drive you nuts, and never advance you career. Those guys all likely paid their dues, and who is to say that a dozen script doctors and ghost writers didn't permanently damage a solid story that existed as written in their original draft? I find it pretty difficult to get angry writers, unless they're complete big shots with a lot of control (Orci and Kurtzman, David Koepp, etc.) because it's rare we get the whole story about the screen writing process.
Plus, you just have to come to grips with the fact that you will be overlooked in favor of total garbage the vast majority of the time.
And I'm at a point in my career where even if I truly hated something, I'd bite my tongue. Even if The Book of Eli sucks, I'd still go on GAF and go, "Great job, Gary. It was really... interesting."
(Interesting is Hollywood code word for "it sucked".)
