completely circumstantial though. sure when you are sitting on the sidewalk listening you can hear them for miles.. but when they are behind you going at a similar speed, its simple physics that you aren't going to be heard, plus human nature.
i've put well over a 200,000 miles on a motorcycle in my life (not a lot by some standards) and never felt the need to put a loud pipe on a bike to 'protect myself'.. its false security. Many of whom i know who ride say the same thing.. the faster you are going the less your pipe makes a difference.
yes, california has the "luxury" of lane splitting, that makes life increasingly more risky, but for the rest of us, driving predictably will save your life more times than driving unpredictably hoping your pipe catches attention
i can't count on 2 hands how many times i've been 'startled' while driving, by some guy on a loud bike who passed me on the freeway, and i didn't even know he was there until he literally was at my mirror.. you simply don't hear them until its too late, and i'm a completely attentive driver (to a fault) while in my truck