I think you misunderstand the point. Useless comments should be downvoted more readily the longer they are as that means they are a less efficient use of pixels. Shorter comments don't need to be downvoted as much even if they're not the best since they're not polluting the screen as much. In general, the best comments will take up some space but I take it there is always some length at which even a helpful comment deserves to be downvoted.