I'll speak up and say that in some cases, aftermarket pipes are chosen for style and appearance. I guess one could say that is a subset of coolness, but in the case of the Warrior, the only feature that 90% of buyers change first was the exhaust because the stock muffler was an eyesore to most owners. If there where no such thing as aftermarket exhausts then it can be summized that Warrior sales would have been poor and would not have been in production beyond the '02 models. I didn't go the aftermarket exhaust for the "look at me" ego boost, it just needed to be improved period. It just so happens, as with most aftermarket exhausts, it was louder than the stock muffler.
The tricky part about the study is, is that it can only afford us data on accidents that "happened" and not accidents that "didn't happen". So one consideration you must open your mind to is IF the question was worded as:
"Is it possible that a loud muffler (on a bike or other vehicle) prevented a catastrophe from happening?"
then, in the famous words of Police Chief Martin Brody, "We're going to need a bigger boat."