Not completely true. On a stock car will you see gains from an aftermarket blow off valve? No, because the stock piece can handle the flow of air from the stock turbo. As you start modding and turning up the boost and, more so, upgrading turbos there will come a time where you will be trying to move more air then the stock valve can move effectively and you will need an aftermarket BOV.

Ever seen a car with multiple BOVs? I know of a few personally that run 2, if not 3, to prevent compressor surge.