Agreed. Seen it on some forums, but this stimulates post count, nothing more.
We even tried to have a post count based group that opened a new board after a certain post count... didn't end well. Basically, if one user got in, all of the content of that board was shared between all of the other users that were buddies with the one who got in

. Eventually, all of the content was shared, even though there were only about 20 users that had permissions to view that board.
IMO, those kind of automatic reward systems don't work. Users will always find a way to fool the system... if someone is genuinely interested in writing relevant content on the forum, they will do it, with or without stimulus. Basically, forum culture is about sharing and you can't expect that people who don't want to share anything (just grab as much as they can) will be stimulated to write meaningful posts for +50 Karma or whatever.