Personally I don't think loud pipes do anything other than roar. Bikes with loud pipes are like Helicopters - you hear the thump but you can't tell where they are till it's too late. Secondly if you ride alot and wear a helmet - sittin on loud pipes drives ya nuts after a couple of hours in the saddle.
LAstly if you ride behing a bike with loud pipes - you can barely hear yourself think! Not for me.
For those of you who live and ride in a rural or country setting this may not be a very important topic. I live in Southern California. Population growth and City sprawl have all but comsumed most of the open land here. That means to commute to work or ride to an open country road we must drive on crowded eight lane freeways. Frequently congestion is so bad that it is necessary to split traffic (to keep the bike and your legs from overheating of course). I found that doing so with stock pipes on made me very vulnerable to cars that drifted back and forth to the line. When I put a set of custom pipes (LOUD) on the bike the problem was solved. If they wereen't looking in the side view mirror at least they can hear me coming. All but a very few drivers will yield and give me space to pass. No longer do I get the "HUA" driver who drifts over and nearly collides with my bike.
Darn right - "Loud pipes save lives"