If they were in a deep cover 2 and you couldn't find a run game that's when you'd have a problem. Devin is right now contributing all this offence needs in the way of a run game. He's fine as the lead back provided he's good with ball security. The Bills seem to improve their running offence as the year goes on (at least they did last year) and the appear to be doing so again. For his whole career he's been a durable workhorse, including his college days. He's not a game breaker but he's good enuf to keep us from having to pass 60 times a game, which would make that deep cover 2 thing a problem even with Allen, Diggs and Davis.
In an ideal world I too see him best as complementary back to a true # 1, but that guy was not CMC, more like the guy in Indy or even the Jets rook. But those guys are hard to find, especially if you don't take an RB with an early pick in the draft. Even as a passing team we do need to run the ball but I think our RBs are capable of getting the job done.