Why is this "5, Insightful": it is one man arguing from experience to absolutely discount another's experience!!!...there is such a thing as a BLIND SPOT you know.
Fair question, and a good point. This is a bit of an old topic by now, so I apologize for being late back into the game, but I think it's worth replying. I did a Google search for statistics, and you know what? I really couldn't find anything to conclusively back either side of the argument. Apparently, I'm not alone; neither could this guy. There is a bit of a priori reasoning, however, that can help sway the argument. As mentioned in the link above, as well as this one, the Hurt report suggests that 77% of multi-vehicle motorcycle accidents are caused by a driver ahead of the bike; most of the exhaust noise is directed rearward, however. Yes, sound propagates in all directions, but it is more attenuated towards the front of a motorcycle...precisely where it would do the most good, if "loud pipes [really] save[d] lives." Furthermore, the Hurt Report summary makes a number of bullet points drawn from the accident statistics. Points 1, 6, 7, 9, and arguably 13 and 30 support the "loud pipes" argument (mostly, IMHO, by pointing out that conspicuity helps to prevent accidents). However, points 15, 16, 17, 21, 22, 23, 24, 25, 26, 33, 34, and 41 support the "loud pipes do *NOT* save lives" argument (again IMHO, mostly by pointing out that rider training/skills and proper safety gear have the greatest correlation to a reduction in accidents and accident severity).
So...do I have any proof to support my position? Not really. I can make a decent circumstantial case for it, but no, I can't really prove it. I can, however, make a case -- despite your suggestion that the Interstate Commerce Clause is a get-out-of-jail-free card -- that loud pipes result in restrictions to motorcycle activities (see the two links I provided above for examples). I'm not a lawyer, nor do I play one on /., so I'll decline a legal debate about whether the Interstate Commerce Clause trumps local ordinances, although I will go so far as to state that I suspect you'd better have some solid statistics to bolster the "loud pipes" argument before you attempt to have your lawyer take on a local judge based upon that reasoning.
Finally, I'll point out one more thing: claiming that you only ride with loud pipes on your bike in the interest of safety is rather disingenuous when the motorcyclist making that claim is riding NATGATT ("Not 'All The Gear, All The Time'" for those unfamiliar with the acronym). You're not wearing a helmet, gloves, etc.? You're going to have a tough time convincing me that safety is what you are really interested in, then. Yes, it gets hot in the summer. That's why manufacturers make mesh gear in colors other than black.