Any open vents (either cam cover vent or the PCV system) will let un-metered air into the system.
Also using breathers will allow the PCV valve to stay open because it lets that un-metered air be sucked in behind it.
When the PCV stays open it eventually starts sucking raw oil into the intakes.
That is very bad for many reasons. A major one is even a minor drop in oil level from full can cause engine failure at high rpm!
Others include worse ring sealing and performance loss. Positive ventilation is a good thing.
