I am voting with the "don't really help" camp. When I am in traffic, I spend lots of time watching the rear-views of the people in front of me. I can usually tell if they know I am around or not. Since I am usually going at least a few kph's faster than them, this is an important technique. I concede that when stuck in steady flows of traffic, where you don't have the option of wiggling around so people keep noticing you, loudness might be an advantage. But, I am an avid user of the horn (not to mention the one-finger-wave) when people start stupid maneuvers in front of me, and I have watched many more close calls with bikes in front of me than I have ever experienced myself. It's like driving a semi: you have to be on the ball steadily because some fool is bound to pull a bone head move in front of you. And I like a reasonably quiet bike. I would and will likely put up with some more noise for a noticeable increase in kick, but definitely not for the sake of just being loud.