I have to disagree. Downvotes (when actually merited) serve the purpose of not only helping sort out which answers are of the best quality, but also (especially when accompanied by an appropriate comment) inform the person who tried to answer the question that his answer hasn't simply been ignored, but has actually be read and found wanting. As already noted, adding a comment telling exactly
what was wrong, or how the answer could be improved is even more helpful as a rule, but a downvote can still be a useful tool.
There are a couple of limitations on that though. The first is that the downvote really is merited. The second is that if it's at all reasonable, you do add a comment explaining the down-vote. Nearly the only exception to commenting when downvoting should (IMO) be when there's already a comment pointing to the problem, so the added downvote is just to help the score reflect the answer's (lack of) quality.