Personally I don't think loud pipes do anything other than roar. Bikes with loud pipes are like Helicopters - you hear the thump but you can't tell where they are till it's too late. Secondly if you ride alot and wear a helmet - sittin on loud pipes drives ya nuts after a couple of hours in the saddle.
LAstly if you ride behing a bike with loud pipes - you can barely hear yourself think! Not for me.