When they "post a bunch of non-spam comments" for the express purpose of defeating the post moderation, it is pretty damn obvious. If it's aggregious, I'll ban people for doing this, especially if the comments are worthless to the discussion.
We've been doing it this way for quite a while (with a few tweaks here and there) and for the most part, it works very well. What you don't see is all the spam that gets deleted before it even gets made public, specifically because this policy is in place.
So I would have to disagree with you that it is a waste of time.