You mean in a situation where a comment is out of order, abusive, or some other obviously bad comment?

I guess if comments were forced on downvotes, let's say "I downvoted you for being a moron" than instead of leaving your own version of "you're a moron" you could add your weight to the first downvote-comment.

So like downvoting with a comment, but kind of adding your weighting to someone elses downvote for the same reason.