Well, it turns 'aaaaargh you're an inconsiderate controlling bitch' into 'your behaviour had x negative effect on me, please ask me before you do something like that again'. Which is a way of doing something constructive with the anger (or whatever) not letting it lie. It's sometimes taken me a few months to feel able to say something that calmly, but I do do it.

But yes, sometimes you need to vent about someone you can't complain to directly, like your boss at work or whatever. Having worked in some really difficult places, group negativity can be really powerful and depressing to work with, so it's best used judiciously. One of the managers at work says he won't share an office with his staff because they need space to bitch about him - a very healthy attitude ;-) and needless to say, they rarely have cause.