We recirc because of the MAF. The MAF is responsible for measuring the amount of air going into the engine, the computer uses this info to decide how much fuel to add. If you dump air after the maf has already measured, then you will run rich. The ecu has decided how much fuel to add based on x amount of air flow, when in reality it only has a portion of x because you vented the rest. As above, a blow through setup relocates the maf down stream of the bov so you are dumping air before its measured