I have long been confused by this - and it appears I am not the only one. Is it for down-voting the poster or down-voting the context of the post. It seems that people use it for various reasons and not always the same. In the past I have used it because I'm unhappy about the context (therefore agreeing with the poster if they are too) then changed vote to something different as it looked as though I was anti the poster.
Would very much appreciate clarification on this. I think it can cause unpleasantness where not intended. At least with the anonymous down-vote one's feelings were clear whereas this can be very much either way. I personally feel it was a mistake getting rid of it (but keeping the anon. upvote) should be both or nothing, can't see the point otherwise.