There are a few purposes of the PCV filter and why to add one to these older engines.
First, it'll burn cleaner. Not only less air pollution, but more importantly to me, no oil out the bottom of the draft tube to burn on the exhuast, dirty up my garage or collect in the bottom of my bellhousing and get into my clutch assembly.
Secondly, using a PCV valve should make the engine a bit more fuel efficient. A nice thing when gas gets to $4/gallon or more. (PS thats been what it costs in lots of Europe, Mexico and central america for a long time only there it is US$1/litre).
Thirdly, it will make the engine a bit quieter and smother running.
Fourth, mechanincally, the valve itself is sort of a check valve- It allow vaccum to suck from the crank when the engine is producing vaccum, ie idling, and when the vaccum is low it closes to prevent blowing straight oil through the pcv valve into the intake, unless I'm mistaken.
In short, the advantages are asthetic cleanliness, reduced odor of burning oil, improved performance and gas milage. Some inline sixes of this era came with PCV valves, they just weren't the throw away kind and had to be cleaned periodically.
