An open crankcase is the most efficient system. I once built a Ford 300-6 to some pretty decent power levels, but didn't have the correct valve cover for it. It had the port for the PCV valve, but not an air inlet, just a screw on oil cap. The engine ran OK, but oil pressure was less than I expected and the power was off a little. When I was able to swap on the correct valve cover with a breather, the oil pressure picked up ten psi, and the power was noticeably better.
Crankcase seals and gaskets are not designed to retain pressure or vacuum, if it's a vacuum you may not see a leak but dirt can be drawn in and contaminate the oil.
Besides that, the pressure will always go to the same point, it's the volume that is the issue.