Generally the PCV is closed at idle, the high vacuum sucks it closed. Then at some point as vacuum drops, it opens up.
Now I'm not saying that there's no leakage, but if the PCV is stuck open it's hard to get a car to idle.
And yes, technically the oil vapors lower the octane rating of the mixture. It seems however, that the benefit of running a little vacuum or at least no positive pressure in the crankcase has more benefit.

My Poly 318 was leaking oil from every gasket. I found the hose to the PCV valve was collapsed internally. Replacing the hose cured 90% of the oil leaks.
R.