Jeez, bud, take a breath and calm down. There was no personal affront intended, and I don't know how you managed to take it that way. I certainly didn't imply that either one of us was an expert, or claimed to be.
I have indeed "bothered to look"...I do a lot of shooting, and I read about it a lot. I've tried most new ideas as I came across them, liked some, didn't like others. I know that this grip is taught at "the highest levels", but when somebody is making money by running a course or a shooting school...just like when somebody is selling any kind of product...I don't automatically believe what they tell me. I need to try it out and see for myself.
Progress...change for the sake of improvement...is wonderful. But change for the sake of change is not progress, it's just change. An experienced teacher will show you something, and will tell you that it might be helpful...under certain circumstances...for you. He doesn't just say "This is better; the other way is wrong!" An experienced student will understand the difference.
Shoulder your rifle, sight down the barrel, and move your thumb back and forth between the two positions. Can you honestly sat there is a difference in muscle tension? If there is, it is...
for me...increased with the thumb in what you call the side-thumb position. And as stated above, when I am shooting a gun that wants to go backward when the bullet goes forward, then I will grip it in such a way as to control it best. That means holding it like a human, not a raccoon.
This isn't a rant; a difference of opinion is simply a debate, not an argument. No need to get bent out of shape because somebody disagrees with you...or with your internet heroes. Cheers!