That three dollar part is VERY important indeed! Without it, excess pressure can build in the crankcase and blow oil out through the path of least resistance say, for example, the dipstick tube or the seals. Leaving the crankcase to vent directly to the atmosphere adds to pollution and that is why it is an integral part of the pollution control system. Years ago, it wasn't mandated and therefore didn't exist. The oil filler cap was usually vented and that allowed crankcase pressure to equalize with atmospheric pressure. That little part has been the cause of countless problems and engine failures, I guess that counts as a BIG problem!! When that part clogs, that's when the excess pressure problems begin. Now, the blowby problem is not a pcv valve problem, but it causes pcv problems; namely a clogged valve or messy residue within the intake system. That blowby is caused by excessive compression leakdown past the pistons compression rings allowing oil on the cyclinder walls typically controlled by the wiper rings to be blown into the crankcase as an air/oil mist. That mist is then expelled out of the pcv system through the pcv valve!

Hope that helps explain things a bit better...