I'm enjoying this discussion, and the RevZilla article on loud pipes! I do feel the subject ranges across polar opposites; loud Harley Davidson pipes vs quiet electric motorcycles, and risk of danger to the bikers vs those around them, pipes or not.
I believe safety is a gradient, ranging from danger to the rider, to danger to those around them. I'm sure many people have read of proposals, in Europe and the USA, to add sound to electric vehicles at slow speed or a complete stop. My understanding is this for the benefit of handicapped people that may have reduced hearing abilities, or sight/body-restricted to fully, and frequently, view active traffic while crossing a road.
I've added a Swedish manufacturer "SoundRacer" system, with dual speakers to my DS, and I use the sounds when at a stop if I see people near the crosswalk. The main use, however, is to get a laugh, and appreciation/respect for a quiet electric motorcycle with similar performance characteristics, from ICE bikers. You can view a 30 second mp4 of the SoundRacer operation here: . I've attached the sound frequency increase line, separate from the wheel hall sensor input line, to the bike high-beams to "blip the engine"