A "vented" engine using only "breathers" must, by definition, have higher pressure in the crankcase than atmospheric pressure, or there's no way the fumes can push through the "breather" filter element(s).
An engine with PCV (Positive Crankcase Ventilation) has fresh air drawn into the crankcase by (regulated) manifold vacuum. The crankcase stays "cleaner" with PCV than with silly "breather(s)", and has essentially atmospheric pressure inside, not above-atmospheric pressure.
On a "closed" PCV system, even if there's more crankcase fumes than the PCV valve can remove, (heavy blowby/low vacuum) the excess fumes are vented into the air cleaner area, so that whether the fumes come through the PCV valve or back-up through the air cleaner, they're still routed back to the combustion chambers and happily disposed-of.
Any engine run primarily at part-throttle should have a proper, functioning PCV system.
A "race" engine, that operates primarily at heavy-throttle, may or may not benefit from PCV.
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