Im not sure boost is controlled at the pulley. Arent S/C making the same amount of boost always? I thought they had stepper motors in them to do this. But either way,....once your boosting and you get off the throttle and the plate closes a BOV would be needed.
My understanding is that the Centrifugal S/C is spinning way faster than a roots type charger much like a turbo moving more air through a pipe to get to the intake.
The throttle body is after the Centrifugal S/C so once the throttle body closes there is a buildup of pressure inside the piping pushing back on the turbines inside the Charger (much like a turbo) So the pressure needs to be vented to remove strain on the fins of the turbine and piping.
Where as in a roots type setup the S/C is between the throttle body and the manifold and a boost bybass valve opens and just sucks air from the intake while not under boost.
From what i have read on the Comptech S2000 system most people are only using a BOV after upping the boost levels.