Sure that's a possibility, but let's go with what we know:
1. They aren't using PA with Jones, which is widely used across the league last, because Jones is so good at it they don't want him to just pad his stays.
2. Zappe is extremely effective at PA, but they still use it because they want him to pad his stats
3. Jones has a QB rating of 70, Zappe has a QB rating of 100.
4. Teams seem to have def figured out Jones weaknesses and it shows in his 7 INT to 3 TD's and Mac isn't doing a great job adapting.
5. Teams might figure out Zappe at some point in the future, so we should play the worse QB.
6. Zappe is doing all of the things NFL teams like to do better than Jones, sees the field better, protects the ball better, moves in the pocket better, etc.
Mac is on a short leash right now. BB and the Pats have invested a lot in him. So they need to give him time. But he puts up anymore stinkers, you better believe Zappe is close on his heels.
But we should go with your theory, Mac is actually doing really well and PA is something be excels at, but the coaches don't want to use it for some unknown reason to us.