So here is my theory.
Newbie comes onto the forum and starts snooping around because he heard about bounties. He wants to make some cash and he wants some cash right now. He starts trying to sign up and realizes he needs to be a jr member to participate. Here comes your first influx of SPAM.
We probably have hundreds of people spamming the hell out of this board with multiple accounts and the merit system does nothing to stop the spam and it may actually cause more of it.
There's some sense in your theory but don't forget the administrators of the forum are working tirelessly to implement new rules and regulations to help better the forum by eliminating spam for good and so doing will return Bitcointalk to it's pass glory.
The merit system was introduced to reduce spam and from the stats so far it has. More additional rules will follow sooner.