Should downvoting be allowed on comments?
If comment to a question or answer is nonconstructive but doesn't warrant being flagged, there should be a way to indicate this to the author by giving the comment a negative vote.
Overall I would not mind if comments could get voted down, at least it would be a better indicator of it's popularity than just counting upvotes, but in the grand scheme of things it is definitely not required.
In my opinion, anyway. Comments are sweet little extras. They might contain helpful or agreeable things (then go and vote), but if they don't, that's totally fine.