I turned off that feature so people can't privately message each other and share secrets, and spread rumors. If you have anything to say, then just POST IT for everyone to read.
There is no reason to keep secrets unless it's about transmutation, or making the stone via the fast paths. Those are the only 2 exceptions to the rule.
Plus, I don't think it's OK that people should be bothered with private messages just because they are members. And usually what happens is one person will private message everyone on the forum, one by one, to spread some rumors or talk trash.