I used to have a Honda Magna which had incredible performance and handling...but that V-4 engine just couldn't deliver a decent rumble. And a rumble is so very important to a rider's state of mind.
It's far more than that. Loud pipes save lives. So that rumble is an important safety feature on any street bike.
I'm currently riding a 1600cc cruiser with Cobra brand straight pipes. Now that's a nice rumble. :-)