There's nothing more polarizing in sports than players with exceptional talent who don't reach their potential. And let's be clear: to have the flashes of brilliance, the days when everything looks right, is not to truly reach that potential. You can't "have it" only to see it disappear.
To reach your potential, you have to maintain it. The debate will rage because the glimpses of greatness create a belief but the inability to maintain it creates dissenters. People would rather have something average and know what to expect than see greatness and never know when they'll see it again.
Predictability can be powerful.
Jay Cutler, in my opinion, has not fully reached his potential, but the argument about his performance always gets hijacked by the questions about his demeanor. He mopes. And that doesn't fit the paradigm of what a great quarterback is.