This may or may not hold water, but in fiddling with the first scene of Winter's Queen, I received this response after writing a mock-up prologue to introduce the villain:
I don't think it demonstrates that the prologue was absolutely necessary. What this basically tells me is "Yes, having a prologue works, and here's why." But a prologue is only strictly necessary if there were no other way to write the story than to include it, and I doubt anyone will be able to cite such an instance