Before the whole "morals" discussion starts again.
The personal conversations are advertised as personal, not private.
They are stored as plain text in the database.
And via MySQL CLI you can view them with a query, or more visually via phpmyadmin.
readPC plugin simply is a front-end gui to this, allowing admins to follow up on reported conversations and review a potential spammer's account.
Clearly it's not meant to go read every conversation between members, users are pretty good at reporting abuse.
readPC by tikibay is great, works wonderful, on 1.0 and 1.1 of XenForo.