I hear what you're saying about blocking schemes. I think a much faster back can get to the edge on plays that M&M can't, and I also think there are holes that they haven't had the speed to take advantage of.
But for me, I think a crucial way to beat teams playing coverage is to harness the mismatch of a great and fast receiving back out of the backfield. M&M are capable, but not dangerous or the type of threat that teams need to game plan. I think Ettienne becomes a security blanket in the passing game against top defenses who can get pressure and cover downfield. Much like Kamara has been for Brees.