I think it's very common and important for people to vote to undo downvotes. I do it. A single downvote can turn something gray and make it appear like an unpopular side of the discussion when it's not controversial at all except in the opinion of one downvoter on a mission or something. At the same time, I may otherwise be stingy with upvotes unless a contribution is especially deserving (or makes me laugh, of course, which always deserves a reward).