OK, so not sure if this thread will survive, it's not scare mongering but I have been asked the question a few times now so I thought I'd answer all at once.
Can anyone read your Pm's:
No anyone cannot but vBullitin after a lot of requests included this feature in version 3.7.3 tso only Super administrators can access and read them, not mods but if a Super Admin gives a Mod access to the Admincp they too can read them but the question is would they be bothered, for the most part NO.
Is this feature used I dunno but there you go..