I don't completely see why any of this is needed. Why can't people just keep their thoughts about a certain post to themselves unless they have anything further to add to the discussion.
well, for me there are many occasions when I wanted to say 'thanks' to some users but refrained from posting because I couldn't come up with something constructive to include on my post. in the end I didn't post anything and ended up feeling rather ungrateful.
the others might have different reasons for wanting this feature, but I would like it to be implemented for this exactly.