I think Don Wilson's logic is flawed. He is assuming the "dummy" prop's ear canals they test their IEM's in are the exact same as all of the general publics. Not true, and when we are talking centimeters, a little means a lot. His logic is also not much different than saying unless you are using reference speakers in a perfect acoustic chamber, it's a waste of money and unnecessary. I would disagree.
Let's not forget when ER4 was designed, the few IEM's that were out there were strictly for monotoring, strictly for tonality and I have said many times over they are great for that. My problem has always been little to no visiceral weight and body (not just bass but a fullness) to the presentation. But I listen for pleasure. To me, ACCURATE is what gives me goosebumps and get's me fully engaged into the music. Also called the sweet spot.
Fast forward 30 years and 99% of the public using earphones uses them for pleasure listening. In this sense they want to hear and feel the music. I think it has very little to do with marketing. It's the sound