For myself, I keep an open mind. I think Marshall & Sanow collected some useful data that's worth reviewing. I also think that measuring penetration into ballistic gelatin allows us to draw some reasonable inferences about how a bullet might perform in a defensive shooting.

One of my instructors wrapped a 2x4 with a kevlar vest and shot it with several different rounds, showing us the permanent indentation into the wood. In the (admittedly for me quite unlikely) event I'm facing a violent attacker wearing such a vest, the "back face signature" of the bullet might matter. In other words, deeper indentations into the wood may indicate a great effect on a violent attacker.