Except when you're in my blind spot, and your pipes are pointing backwards, I still won't hear you until you're in front of me and I've already seen you. Cars are incredibly quiet inside these days.
I also agree that you don't have to make your bike EXTRA loud to be noticed but there is a perceived safety in sound. And if majority of the sound is exiting towards the back, there might a point to be made that the decibel level should increase to "hopefully" make the person in front of you aware.