I think I just have a different set of security assumptions than you do.
It is not about security, it is about trust.
Luckily, being a member is voluntary and no one forces anyone to join and sent PMs.
This is true, if I know I am agreeing to this.
Even if you trust theymos, there are 3 other admins, the servers are hosted by Mt Gox, and there's always the possibility of hackers and subpoenas which trust won't save you from. Posting private information (especially if it could incriminate certain people, since we're talking about Pirate that's possible) in plaintext on a forum, pm or not, is asinine. If you value your privacy that much you should take steps to ensure it yourself.