Its done like that because the engine management system cannot cope with a blow off valve. Why would they design it to do something its not supposed to do?
The MAF sensor measures the quantity of air thats entering the turbo, and uses that value to calculate how much fuel to inject. When you fit a BOV, including your splitter, every time it dumps a quantity of air thats already been measured by the MAF sensor gets dumped and never reaches the engine. The problem is the engine management system has no way of knowing this air is no longer present, and injects the fuel anyway.
This has the result that every time the valve dumps (ie a gearchange or lifting off etc) you get a momentary rich mixture, which causes flat spots, stalling and generally crappy running. It can also affect the fuel trims which control the overall engines fuelling, as the ECU will try to compensate for the richness by backing off the fuelling.
Simply fitting a cone filter with the OEM valve will give you a bit of a dump noise, but at the same time will keep the ECU perfectly happy.
