Thanks! So, just to be sure:
Since most of my users PM's are mails between 2 users, this means that when both 'remove' themselves, then the pm is gone?
Indeed so; as long as all users are unsubscribed from the topic (regardless how many there where, you can have any amount of users in a pm, from 1 to whatever the limit is in the control panel (admin's subvert the limit because they're just that cool) the private topic is purged.