Well, the PCV system is designed to alleviate pressure buildup in the bottom end of the engine block. It has to evacuate the air someplace. The air being evacuated, however, contains a lot of pollutants... so it's more ecologically beneficial to route that air back into the intake system to have another go at combustion. An open-air setup just dumps the pressure into the atmosphere.
They both provide effective crankcase pressure relief, but the open-air filter setup pollutes in the process.