Many more biker owners than Harley owners put loud pipes on their bikes. Very popular with the sport bike crowd and I've even seen Goldwings with 6 inch pipes on them. My '01 Kawasaki Voyager XII came with cobra pipes and I hated them. I would always end up short shifting.Don't even get me started. I just got back from a first great 2000 mile trip on my Versys 650LT. A friend invited me along. He also had another friend along whom I'd never met. That friend rode a Harley. My friend was on a Nomad. His friend rode in the rear for a reason!
After all the Gold Wings and all the ST's I've ridden, I just can't stand the noise. And it has never been proven that 'loud pipes saves lives'. It's a Harley myth.
Anyway, back on subject, when I was in a rental car in California on the interstate I decided that there was some merit to loud pipes. But that is only because of lane splitting and only in California. I could hear a bike even though I didn't know where it was. This kept me holding my lane position. The onus is on the rider to not linger in the blind spot because after a certain amount of time I would assume it is a super loud bike somewhere else on the road.
So other than California and the race track, I see no place for loud bikes, cars, trucks, etc.