I'm not sure that having a horde of posts trying for '+5 Funny' really helps their discussions. On HN, you tend to get downvoted for posting lame jokes, and even some good ones.
That's not the way conversations tended to go on Slashdot, from what I recall. It wasn't like reddit. Furthermore, because Slashdot's system differentiated funny from other kinds of ratings, you could specifically suppress posts rated funny, or raise posts rated Troll, etc. This functionality was built into the interface.
