I thought this was a really insightful video. I completely agree with Scott's assessment. I believe this wide acceptance of the thumb over grip is a symptom of a lack of fundamental discipline and/or as he describes early on; a poorly fitting rifle. Ergonomics matter and the grip is usually the first ergonomic interface that we interact with. It aids in the CONTROL of the rifle, which is what thumb over shooters intend to enhance, albeit counterintuitively. Coincidentally it is usually the first ergonomic interface that we're willing to disregard.
I also agree with the "thumb dance" explanation, but would like to take it a step further. The AR/ Carbine was intended to be a fighting rifle. A rifle that was meant to shoot move and communicate. The "thumb dance" not only adds time on the clock in a competition. It also inhibits the users ability to "Shoot, Move, And Communicate." The rifles originally intended purpose. By choosing a thumb over posture we are decreasing our ability to control the rifle while moving.
What do you think?