What's with the threads that have a main topic, but then a random poll? There have been about five or six threads with something like this in it. I'm not gonna dig `em up, but the most recent is that sleep thread in brewsters.
As stated before: It gets more idea and conversation flowing in the thread. And more conversation = more posts, which means more success in the thread. And the thread posters don't want a dead thread, that'd be bad.