I dont know how much they actually save lives, but from a logical standpoint you can't deny the claim that something that makes more noise will get more attention than something that makes less. Look at the Prius controversy...those things are so quiet at certain speeds that the Blind community is pushing for noisemakers and just the other week some kid got injured when he got hit by a Prius he didn't see.
quiet bikes have almost no exhaust sounds when they approach from behind. I've driven cars in LA and been stuck on the freeway. When lanesplitting past you, you can hear a harley a car or two back. You notice crotch rocket abouts a quarter of a second after you hear "vwwoomm" go by your window, ten inches from your head. (Common reaction is usually a
and some choice words)
PS- low frequency sounds travel farther than high ones.