Whether McFadden is a "special back" or not isn't the point. The odds are extremely good that he'll be the third best RB at running in a zone blocking scheme. But he was likely not as good as Christine Michael either. He is invaluable however, because he's the best RB we have at picking up blitzes. He can be used on third down, he can be given the load in the event that Zeke gets hurt. The coaches can trust him to pick up the blitzer and protect the QB, which is the most important job any RB on our team has. And before anyone says running is more important, the coaches don't think so. That's why McFadden was out there getting carries every week when the odds are very good that Michael or one of the other backs could have done at least as well running the ball, and without the shift to more of a man blocking scheme.
McFadden can pass protect, catch and run. He can handle 25+ carries a game. I think those things virtually guarantee him a roster spot.