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While I agree very much with the general concept, I think for the most part the person stating the myth sincerely believes it. As human beings, one of our innate failings is that we tend to believe what we are told. If we somehow hear one of those myths, we repeat it without thinking, no matter how silly it seems if we give it some thought.
Oops, I forgot Thal's favorite: "You'll die if you dive the Pacific with a weight integrated BCD!"It's just another NetDoc excursion into the realm of logical fallacies.
Its passed down from Instructor to student much like "Computers rot your brain!" People with agendas and strong opinions often stretch the truth and reality to make their point. They may or may not fully believe what they are saying, but they want to win the argument. Currently we have one individual bent (soon to be literal) on proving that bounce dives are normal and a good idea. While some would have us simply censor such drivel, it serves as a great sounding board for responsible divers to discuss how to dive safely and within sane limits. Without such candid discussion, these myths would have a far wider audience. Reasonable people have no problems discerning reasonable discussions and rejecting unreasonable claims.
So you know, I had already PMed Thal to let him know that it was just a bitOfun at his expense. I told him that so I didn't accidentally start "World War Thal".... which I also thought was punny. Hey, I laugh at my own jokes only cuz no one else will! 
Me too... and I have found that it has made its ways to the Keys, but the instructor making the statement had no idea of its origins and was using it all wrong. The funny part? When I told him the origin was UP and what it was in reference to, he told me I was wrong and should do a bit of research to get my facts straight.
Guys like this are always priceless to talk with and he regaled me with lots of stories about his daring do and how he single handedly saved countless students. I almost pointed out that properly trained, students shouldn't need saving, but why? Its impossible to reason with unreasonable people. The point is, once a pithy saying expands past the original circle of influence, the inference shifts a bit. Like the bit about giving your regulator to a fish. The first instructor I heard it from stated that narcosis was precisely NOT that. His divemaster that became an instructor told one of his classes the exact opposite. After his class I pulled him aside and told him that wasn't a good way to teach narcosis. He didn't believe me either until he called the instructor trainer and got straightened out. No, I don't do that often, but Tom is a good friend. Friends don't let friends teach stupid. 
