I'm torn on if I think that they actually save lives. Yes, they are louder, so people that aren't already distracted will likely hear you. But, how about that guy who is already talking on his cell phone with the radio cranked? It's not going to do you any good for the people that are oblivious (or just plain stupid).
Another negative, I feel, is that you can't hear other cars as easily. When you are riding a quieter bike, you can hear people coming up behind your a bit easier. Not so much with a loud bike.
Too loud is waking up your entire neighborhood as soon as you turn on the engine...