“I disagree.” Can I downvote you now?

That’s what not requiring comments prevents: useless comments that are just posted so people can downvote. So now your comment section is flooded by “I disagree” comments and you can’t downvote those without leaving a comment as well. It’s a self-serving cycle.

You reverse causality, you shouldn't use a downvote when you disagree with someones opinion to begin with.

I took a karma hit of -4 on the comment above just because people disagree with an idea I proposed.

@dang has said downvoting for disagreements is ok. I can’t remember where, but he has.