I always try to leave a complaining comment when I downvote, except when the reason is already given as a comment (then I'll upvote that). I guess this is a best practice scenario though.
A one-paragraph answer, with bad writing and no, little or redundant information, however, I do not think deserves this comment. In these cases I often flag and downvote, too.
I have to admit, that I only comment, when I think a post can be salvaged through editing. (In that case I also want to be notified to retract my vote.) Some posts just need the down-vote to get them into the deletion queue.
This toppic has been deeply suggested and extensively been discussed on meta.se. From that post I drew the conclusion that the majority does not want to be forced to leave a comment. I would not want to be forced to do that either.
Also if a comment is mandatory it may lead to more cleanup to do, to filter out complete gibberish, that was only entered to fill the necessity.
Another point was mentioned by jonsca, that the unmasking of a downvoter may lead to revenge downvoting, which should be avoided. This may not be a problem on our site, but apparently other networks have this issue.