I agree that actual words explaining why you gave a downvote is far more beneficial than simple silent shunning. I strongly endorse giving useful feedback whenever you downvote on a post here.

However I disagree that the usual kind of youtube-comment vitirol is useful data. Profanity-filled ad-hominem does not give you useful information to update upon. I'd rather five silent downvotes from a community whose opinions I trust than five unhelpful youtube comments. YMMV