I have never bought this argument. Motorcycle pipes point backward. On the highway, you can't really hear them until they have passed you and are thus already in your field of vision. If the point is to alert drivers that you are on their blind spot, why not direct the sound forward?
Direction of the pipe might be more efficient on that front but it doesn't mitigate that it DOES help. I can hear cars behind me that are loud and their loud mufflers are pointing back too.
You might never have 'bought' the argument but direction of the pipe doesn't take away from the fact that MOST people DO hear them as they begin to pass you. Except when they zip through going illegal mph. That's something that...baffles me. Why zip through WITH loud mufflers. They don't seem to be too bright but...fate comes for those cyclists fairly quickly.
