For some reason comments are often more interesting than the articles themselves, and people never think of monetizing their comments. Somehow karma is enough of a reward.
If I had to watch an ad for 30 seconds or pay $0.25, the proceeds of which would be distributed to commentators by proportion of karma they earned in the comment section ... I probably wouldn't bother.
It doesn't make much sense to me how I arrive at these utility valuations though.