I used the thumbs down and thumbs up quite a lot while it existed. I used thumbs up for posts that made me laugh or were particularly helpful and in particular for new members who I thought were trying to add something to the discussion threads.
As someone who received a thumbs up rating at one point of 20 that dropped to 17 it did at first bother me and then I thought who cares. I always try to be polite and helpful and if that's not good enough then tough.
My main reason for using thumbs down was because I think sometimes people can be quite snippy in their replies to new members in particular. I personally think there is no need for this. There is always a nice way to point someone in the right direction or advise them better how to use the search facility or the forum in general. We were all new members once and not everyone is familiar with a discussion forum.
I also meet a lot of people and many read the forum but say they would never post and the main reason for that is that they are scared about how they or their contributions will be received based on how they have seen some topics develop. I always try to encourage people to post and that what they have to say is worth sharing.
Overall I believe the forum does an excellent job and that spats between members and unpleasant personal comments are fairly rare and new members are warmly welcomed into topics these days
