EDIT: Added my thoughts about "downvote as quality info"; removed the part about opinions (thank you Manishearth); explained better the proposal.
The question is already in title: do you think that down-voting is really useful for any Stack Echange community?
In my opinion down-voting
misses its primary intention: does not educate the user. I think it's more useful for the user you would down-vote and for the entire community to explain what's wrong in the post with a comment.
(new) As pointed out by Yannis and Manishearth, downvotes are useful to signal bad posts, but you can do the same upvoting good posts. This is even more so for answers, since you can also vote to close them.
Down-voting is like giving a silent slap. Commenting is like admonishing. It's not fair that a questioner and an answerer must explain clearly what they write, while a down-voter can simply click a button. Personally I down-voted only two times, when the post was already commented by me or someone else as not good and the author had not changed it.
Yes, Stack Exchange sites will suggest that you should add a comment, but from my (little) experience people that leave a comment before down-voting are a minority. For example, a question of mine had a bad answer, and I commented it to explain why it is bad; in the meanwhile the answer was down-voted without any comment. After a while the answerer deleted it. I do not think that the down-vote was particularly useful. And if I can be completely honest, I'm a bit angered by the fact that the answerer can think the down-vote was casted by me :D
Furthermore down-voting is particularly misleading for sites like Meta Stack Overflow or Programmers or something, since I noticed that people on these sites tend to down-vote a post because they do not agree with it instead of because the post is badly written.
I think a more useful method would be to allow users to check a "Down-vote if not edited" box. If such a comment is left and the post is not edited after X minutes, a down-vote will be automatically cast. (changed) If the post is edited, the commenter will be informed and can down-vote if (s)he don't like the edit. In my humble opinion a system like this will conjugate the quickness of down-voting with the fairness of commenting.