Other than making your engine dirty, with
oil leakage, I have heard that some times, especially with the FE design, as the blowby gases escape through these breathers, with the gasses passing up from the crankcase to the heads, through the openings where the
oil drains back. At times, if there are enough of blowby gasses, and the
oil return passages are too small to handle both the blowby gasses and the returning oil going in opposite directions, you can end up with excess oil retained on top of the heads, being retained by the blowby gasses. If you run solid valve covers, and the engine blowby gasses vent through the intake, then the oil can drain easier from the top side of the head back to the pan.