That's my point about a pass, a completion is good and incompletion is bad. But there are many datapoints that effect the result and I don't understand how you collect and assess them to come up with a value added result beyond what could be determined by a person. Not saying it can't be done, just seems like a larger set of variables than baseball because of the team nature of the sport including a playbook etc.
Also for example, a few years ago Kavika (sp?) Mitchell was called out on twitter for not making a play in the pass defense. It certainly looked like it could have been his fault but according to him it was not his responsibility. How do you assess a player when you don't know the responsibilities and how well they were followed?