The problem is that bad quality comments are cheap and good rebuttals are hard. The downvote mechanism does improve quality and people usually know why they got downvoted even if they won't admit it.
My (admittedly short) experience with the platform is no, people will rush to down or upvote on the basis of general sentiment, maybe scanning the first sentence etc.
Requiring a comment with a vote will only lead to tendentious arguments over voting, and such discussions are always boring, low quality and tangential to the subject. This is why downvoting without commenting is preferred here, and discussing downvotes is explicitly forbidden - silent rebuke is often the best way to separate signal from noise without doing further harm.
