Is there a policy to encourage people to make comments when they give downvotes? Commentless downvotes do very little to improve the site. If something is wrong, you should explain what about it is wrong.
Commentless downvotes do very little to improve the site.
Oh, but you're wrong! Voting is at least - and probably more - important than commenting when it comes to improving the quality of the information presented by the site. Up-voted questions are given more visibility, while down-voted questions are given less. Up-voted answers display (by default) near the top of the list under their question, while down-voted answers display near the bottom. And users with a history of posting poor questions or answers can be restricted from further participation.
Comments... Hmm. Carefully-crafted constructive criticism can motivate a receptive author to improve their post. But not all voters are sufficiently diplomatic, nor all authors sufficiently receptive of criticism, to make this ideal outcome a reality. And not all down-votes require comments: the criterion for down-voting is simply that the reader did not find the answer "useful" - this doesn't necessarily indicate a problem that needs to be stated in a comment, any more than a comment noting an answer's usefulness needs to be coupled with an up-vote.
Ultimately, comments and votes are two different forms of communication, with different intended uses and different audiences. The system does encourage you to leave a comment when down-voting, but if you don't have anything worth saying, then... don't say it.