If you simply received a karma point for making a submission or a comment, then I imagine we'd receive quite a bit more spam. Instead, we're all put on the spot to make valid contributions. This is great, however, there isn't really a forum for users to dispute the lack of merit when downmodding occurs. As far as I'm aware, no web community has that sort of democratic judicial arena for disputes, and it would be nice. Something to think about...
That means it pays to be a karma karma karma comedian.