be capable of ever executing well -- the read option. Not everyone is capable of making those reads consistently well and then executing (run or pitch). Golson does throw better right now IMO. However, because he isn't a threat to pitch the ball when he runs, defenses do not have to defend that possibility. Thus, they can defend the pass better (by not having to devote as much to defending the run each play).
While MZ may not be as crisp at passing, that may not ultimately make him less effective as a passer because if they defense has to strongly consider he might run, handoff, run and pitch correctly or pass each play it is harder to defend all really well -- especially if it isn't 3rd and long at the time. If Kelly really were to mix things up on first down, and when he does pass resists the urge to go deep on first down where odds are it is 2nd and long, then MZ passing will catch them more by surprise and an average throw may still be completed because the defender is giving a larger cushion.