I think it would be very useful to be able to show disapproval for matters that need disapproving, whatever those might be is strictly up to the person with their finger on the disapproval button. We all like being able to have an outlet for our opinions, hence the age of social media, so why should it all be one sided? I think people on social media are capable of handling the power of a dislike button. After all if you dislike what someone else disliked than you can always just click dislike and move along.
It doesn't have to be used negatively, but I am sure some people would. I dislike that...do you have a button somewhere on here for that? Oh, but of course there is....it's the "vote down" button. See, we already are handling this kind of power just fine.
By the way, I have never used the vote down button. Normally if something is not my favorite, I simply look for the positive and comment on whatever is good. Why harp on the negative?