Rear crank seal is one of the most common leak points from an engine that passes its first few thousand miles without leaks. Once you get an engine sealed up, it should stay that way for quite a while. Excessive wear and (if you run a closed or insufficiently vented crankcase) crankcase pressure will eventually create leaks, often starting with the rear seal.
There's a lot of opposition to PCV just because it's "smog equipment" - but in all truth, every engine should have some form of crankcase venting, ideally assisted a little by a vacuum pull to force the venting under all conditions. Reducing crankcase pressure saves seals and gaskets, and getting the hot vapor out of the case (and ideally burning it in the cylinders rather than just venting it) prolongs
oil life.