Loud Pipes Save Lives is the justification for Harley owners to have loud mufflers on their motorcycles.
There is a bit of truth to the statement. Motorcycles are harder for motorists to see. If the bike has a loud muffler, there is a greater chance for the bike to be heard and hopefully be seen. If one can hear the bike coming, chances are one will look for it.
On the other hand, Harley riders like to point out crotch rockets have quiet mufflers and get into more accidents. It may or may not be true that crotch rockets have more accidents, but it has little to do with the mufflers. The major contributing factor to motorcycle accidents is rider inexperience. Regardless of motorcycle type, riders with less that one year of experience have a higher accident rate. Regardless of motorcycle type, riders who have not taken a motorcycle safety course have a higher accident rate.
Loud mufflers on a Harley are there to draw attention not only to the bike and to the rider. I have yet to meet a Harley rider who doesn't want to let you know that they are on a Harley, The One True Motorcycle. The Loud Pipes are not only there to let you know that they are coming, but more importantly, to draw attention to themselves.