It's just basic common courtesy to say not just that you dislike something, but why you do so. (It's fine to say something is good without going into details, however.)

I agree with you; in fact I generally do write informative comments to nodes I downvoted. But I doubt that that would be a practice that could be enforced. Just consider nodes of trolling content or otherwise good-willing but definitely annoying users to whom other monks already explained what's wrong with their posts. One may just want to downvote them too - if he feels the content is so bad...