Well, you said it… McFadden had the edge last season. For whatever reason, he did, and for that reason he is the starter until somebody else takes the job from him after all that's the fair way to approach it anyway. You don't give the job to somebody you just signed when a returning vet is a 1000 yard runner from the previous season. As I said, Morris will have every opportunity to take the job from him, and if it works out that way, then so be it. My only reason for helping McFadden gets the majority of carries is so that both backs can be effective. McFadden has shown (last season as a matter of fact) that when he carries the ball a minimal number of times he's not very effective. On the other hand, when he carried the ball 15+ times last season, he was extremely effective. Morris, on the other hand can be very effective on 10 carries a game or there abouts. Neither back is good for 30 carries a game, so it's imperative for them both be able to run the ball effectively. That means it's necessary for McFadden to get a good dose of carries every game.
Another thing you have to remember, Morris has never been much of a pass receiver. I have a friend who's a Commanders fan, who says he has "brick hands". If he's not reliable as a receiver, that's going to limit his number of snaps anyway. That's one more reason I think we'll see McFadden getting a good chunk of the playing time.