And it's why it exist.
It's a not visible part of popular forums, but there's people who try to be smart and, instead of spam posting by starting thread in all parts of the forum, are spam PMing members.
It's more insidious, because invisible from the moderation until someone complain about a PM he received. Yet it's as annoying for the members who receive such PM. Reason why there's a post limit before one can send one.