The bottom line on Arrington: His official role was of third-down back, but he never played on third down unless he was running a route because he couldn't pass block. The role went to Edge, who can still chip with the best of them.
Arrington played in a tight formation, play-action offense in college. He wasn't asked to sit in shot gun and pick up the extra blitzer. As much as he improved in his time here, I suspect he's hit his potential and his productivity with the Cards would not equal what he's going to make elsewhere (Denver, wherever).
For those looking at the draft to fill this position, McCoy wouldn't be what I would call an immediate solution, either. He played in a bass-ackwards offense in Pitt and didn't learn stellar pass blocking critical to the offense. Moreno might be, though. He did a fair job and Georgia used a lot of shot gun in their offense. If Javon Ringer drops into, say, the fourth or fifth round, there's your sleeper back who can fill that role probably better than Arrington just by showing up. It also gets Edge off the field on third down.