It could probably be argued that needing this plugin is actually a symptom that you need to rethink some part of your UI. If an element is important enough to be pointed out like this, it should perhaps feature more prominently in the layout.
Just to explain why I downvoted you, since you're new here: on HN, if you agree with a comment, you should upvote the comment. Writing a comment whose only content is stating agreement is very frowned upon.
He wrote the comment for the same reason that you wrote yours. Comment scores are hidden and the meaning of an upvote is hidden both from the commenter and other users.
With hidden upvotes the only way to show assent is to comment. Actually as a hack for no comment scores I think I could come to appreciate a string of "agreed" comments as a visual indicator.
