Actually Gado is taking all of the first-team reps right now and when Ahman is healthy again, he'll have to battle for the starting position. McCarthy has said there will be open competition for the starting RB spot, and no matter who wins, they'll still take the RBBC approach.
Taking Ahman right now is awfully risky. If, however, he returns to form, wins the starting job, and is able to convince McCarthy that he can be effective with 25 carries a game, then he might make a good pick. But right now there are too many if's.