It's about putting up some resistance to the overwhelming trend for forum sites to devolve to Slashdot. If you wan't Slashdot, go there. Most of the same subjects are covered.
I think this is only a problem if the humor isn't funny enough. I don't know if I hit the mark here, but if I make unfunny jokes I hope I get called out on it (or at least see it reflected in the vote totals), and that'll definitely regulate my posting. If we somehow manage to make enough good jokes to drown out on-topic discussion, I'm going to be very impressed more than concerned I think.