Some call PMs Personal Messages, others Private Messages. Either way they're no way private as they can be read in PhpMyAdmin.
Personally I think this is good to have. I once had a user who had an additional 9 accounts, and used those additional accounts to bump her own fanfictions, and give herself reputation power. One of these fake accounts "stole" the originale account's fanfiction, and well, yeah... Lots of odd stuff with that person. My point being, I could have saved a lot of time having this hack.
Whether or not you use it on all your members, well... That's up to you. And to use on troublesome users, I don't see why not.
*installed* Thanks for this hack!