While it is often more useful to the site to accompany a down-vote with a comment this is not required.{*}
Down-votes are just as important as up-votes in making this site useful. Physics has it's share of bogus information and, indeed, of people pushing nonsense for their own reasons. A visitor coming to this site and finding material that she knows to be crap accepted will not be impressed, and visitors who don't know may be led astray.
In the absence of down-votes you can signal the difference between only two levels of quality (say between good and not-particularly good). With down-votes you can now signal three classes of material (say good, unremarkable, and poor). More discriminating power makes the site better.
Personally I find that down-votes cast due to a single detail in an otherwise good post are the best candidates for comments because the writer can easily fix the problem an the caster then rescind the vote and perhaps even award at up-vote instead. Note that this process contributes to making the site better.
I also sometimes comment when casting a down-vote on posts that are unrepairablly bad in hopes that the author will prefer to delete them---thus making the site better.
In the particular example you selected the OP wrote a question that a number of people found to be rude. Perhaps they cast down-votes out of pique. Well, that's what happens. Vote are plenary as long as you don't focus on another user as user and don't maintain sock-puppet accounts.
I should, perhaps, remark that this consideration do apply to questions as well as to answers. Visitors to the site's main page will see questions before they see answers and if they see a lot of poorly written, ill-thought out, derogatory, questions they may conclude that this site has little to offer them before they ever see how we answer.
Users with the (quite modest) required rep who think that they can improve questions are encouraged to do so (there is even a little bit of rep in it until you reach 2000), as this is a better way to improve the site than casting down-votes, but down-voting is still better than allowing a bad question to pass unremarked.
One final remark: please do not cast votes on the basis of the current score (either pile-on votes or pity votes), but restrict yourself to making your best evaluation of the post.
Read the tooltips for the voting arrows, they provide clear and simple guidance on when and how to vote.
{*} Feature requests to require this have been turned down on the mother meta more time than I care to count, and for good reason.
