I filter because, while truly believe in an individual's right to espouse whatever belief they wish, I also reserve the right to remove myself from the presence of utter stupidity or wasteful bullshit.
In other words, if I feel the poster has nothing useful to add to the conversation, I filter so I'm not even the least bit tempted to be drawn into what I consider to be a whole waste of space.
Besides, then I have to scroll less and that makes for a tidier bread.