And I thought disagreement was the worst reason for downvoting. I thought that you should downvote comments for low quality, when they add nothing but flame potential to the discussion.

Downvotes let a comment fade to grey until it is invisible. Is it so hard to see why someone would see this as punishment?

Why would I want to make counter-arguments vanish? A discussion where all visible comments promote the same opinion would be a boring, useless discussion. Actually, I tend to upvote opinions that I reject if they are outlined in a valuable manner. It is in my best interest to know the best points for all sides.

And OP, as I understand it, wants to make this more visible to downvoters: that disagreement isn't actually in the canonical list of downvote reasons.