I'm pretty sure the sorting of comments has nothing to do with karma of the poster, just the votes on that particular comment, and how old it is - similar to how stories are ranked on the front page.
It's making a game out of it, which gets a small subset of users maniacal about their Karma score and thus submitting tons of good content. PG = Crafty Devil.
I kind of like that idea. It's useful to keep track of what people have generally agreed with in a discussion, or to move things up the front page, but significantly less so for the site as a whole.
