Not to try and shoot you down or anything, KNI, but I disagree. I mean, you do have a good point, but I think writing is more of a way of putting your emotions to words in a different form and letting the world know what you think. When I'm angry, I write about a character who's angry, and finds a way to resolve the problem. If I'm sad, I write depressing stuff.
As a more personal example, I wrote 'Forget the Mountain', not when I was angry, but after I'd calmed down. I personally didn't like how the story turned out, which might be what you were trying to get at, but I certainly wouldn't say that my anger ruined the plot. Thinking back, I'm tempted to say that writing helped me simply to forget about what I was angry about, and move on with life.