I think you assume wrong. People with piggy back loans don't pay PMI and my understanding is that subprime borrowers don't pay PMI either. That is why there is a problem.
Okay, what percentage of subprime loans also had piggybacks? How would it be possible for a subprime loan NOT to have PMI when my wife and I, with very good credit, would have to get PMI?
Are you a mortgage broker or have you just heard these things?