Besides the obvious spam problems that could arise from the PM system being activated (This is why I just list my IM screen names in my profile so they show up in my header bar of every post I make,) this could also add a large amount of overhead on the database and the bandwidth, putting undue strain on the server hardware and affecting overall site performance. This would also likely make the nightly downtime take more than the typical hour, possibly doubling that time, leaving the site accessible for only 22 hours a day.