My experience with it on trac and uservoice is that it's an easy way to filter out things that exactly one person cares about, and not much more. The main benefit is that it reduces the number of spammy +1 comments you get.
ProTip(tm): HN does this when you try to reply too fast. You can click 'link' to get around it, though. :)
I never saw that! Thanks. Somehow that just escaped my vision.