It's fair to question the value of the ability to diagram plays and defense however I will disagree with you and here's why. The ability to recognize plays, assignments et al enhance one's ability to know what a player was SUPPOSED to do on a play. And the more you know about what a player is SUPPOSED to do the better you will be to gauge how well they did it. Again, nothing is 100% but I am a firm believer that the more knowledge you have in an area the better able you will be to accomplish what you are trying to accomplish. If I can recognize that a secondary is in cover 2 and the corner follows his man across the field and the other team throws to a wide open man in the zone he abandoned then do I sit there and go "well look he covered HIS man?" or do I recognize that he blew his assignment and allowed a big play because of it. Knowing the X's and O's is important. Yes there is more to being a GM than just that but I think it's big. If all the GM does is execute the playbook handed to him by coaches and scouts then he is at the mercy of said coaches and scouts.. So he ends up giving too much weight to the voice of a Rob Marinelli, who while a very good DC, was a terrible talent evaluator. Now Jerry probably didn't know that when he first hired Rod.. but he should not have taken as long as he did to figure it out.
I know there is nothing I can say that will convince you of what I am saying.. I'm fine with that.. It's what I believe and it's not going to change.. I'm not going to go be an unpaid intern for some team and spend the next 10 years trying to get to be a GM. So I will never be able to prove any of it. But we all get to believe what we want to believe in life.. This one works for me!