The pcv valve serves two functions, first, as an emissions control. Crankcase vapors would be a huge component of air pollution if it weren't for the pcv. In fact, the pcv was the first emissions component installed on cars.
Second, the pcv helps with oil longevity. Inside your crankcase you have a nasty mix of fuel vapor, water vapor, and some acids that form in the process. Without a way to escape, they condense and fall back into the oil. Your oil goes to crap very quickly without a pcv.
The way the factory incorporates a pcv is to keep a constant flow of fresh air through your crankcase under vacuum to eliminate these harmful vapors. Air enters from the tube behind the aircleaner, through the crankcase, and then back through the engine to be burned. When properly set up there is no performance detriment to having a pcv valve because at WOT there will be no oil vapors being consumed by the engine.
The pcv valve acts as a check valve, and a restrictor all in one limiting the total amount of airflow the engine can pull from the pcv. This is why there are different valves for different engines. The orifice size and weight of the puck are taylored to meet the engine's vacuum signature. A lot of us in the TR world add a secondary check valve because the pcv tends to leak. In this case, the pcv is just there playing the role of restrictor. If you were to just install a check valve you would have the equivalent of a large vacuum leak.
If you doubt the necessity of a pcv on a street car, i suggest finding someone that has one with a moisture seperator inline. You will be amazed at the fluid that comes out of these things. Its a milky mess of water, oil, fuel, and God knows what else that would otherwise be right back into your oil.
