No idea... all I know is that when I try to comment to a "screened" post, I get "Access denied, you may not post replies to this users posts, this user has blocked all comments from non-friends"

Have you used the Friends Screen? Have you ever received some srt of "There is a reply to your post waiting to be approved" message? And if so, doesn't that sort of negate the first argument wherein you would use the screen in order to avoid having to read comments frm the asshats who were harrassing you?