You don't need one on a race car or at least not one for short races, drag races, but you do need one on the street for compressor longevity, better mileage, and better driveability. A pressure activated pop off valve serves better for drag racing applications. On low boost application a BOV is not necessarily needed but your engine will operate better with one. However, if your blower is too small and can't make up for that release in air pressue quick enough then no BOV should be added.

I've run with and without and have seen first hand how much hotter the engine gets with no BOV even at lower boost levels. The gas mileage also goes to hell, without the BOV, because the blower is always trying to compress the air and can't free spin as it can with the BOV. Your car can operate under vacuum conditions, BOV present, and not cause drag on your engine unecessarily and you will get better mileage, better driveability, longer compressor life, and ........