The PCV system draws vapors out more efficiently thatn the road draft tubes did. Over time the unberned hydrocarbons and exhaust gases including water vapor built up and diluted the oil causing sluding and acid formation. The PVC system used manifold vacuum to draw in fresh air, diluting the combustible gases in the crankcase and burning them in the engine. The system yields cleaner air and less oil contamination. It is also said to slightly improve fuel efficiency since you are burning those otherwise unburned fuel vapors.
By open, I assume you are talking about a PCV system with air pulling through the vented oil cap instead of the old road draft tubes. One advantage of the closed system that pulls air from inside the carburetor filter, is that unfiltered air is not allowed into the system. As I understand it, there are no disadvantages to running a PCV system.