It's really not too tough to find the time to make 10 constructive posts in the forums. We don't need a forum that would be nothing more then a spam dump to allow people to bypass the 10-post rule, which is in place to help ward off spam bots.
I think it has to do with blocking spammers... you know the "click this link and gets viagras!!!!" type posts.
We had a lot of spammers / SEObots using signature advertising -- put a link in the sig and post lots of bland posts to increase pagerank with backlinks.
This helps to thin them down, at least.
And yeah, the other point is to encourage people to post somewhere constructively. There's such a wide variety of topics between various wow forums and off-topic forums; there must be something nice to comment on.