@Tomkitten,
Tomkitten, here is a background thread on the thumbs up and down "buttons" - which occur for both threads and individual posts within threads.
http://able2know.org/topic/120756-1
Personally, I use the thread thumbs as an interest indicator and kind of a "clean up" of my list of threads to view.
I use the individual post thumbs, when I do, to thumb up a well expressed point of view, or, much less often, to not have to look at a super rude one.
There have been some modifications since that thread. (Yep, and intrepid posted about one of them.)