I'm okay with this signing. I still believe if West is healthy, and that's a big if' that he's backing up Rondo. Considering how little he's played, West has looked pretty good when he's out there. He has a desent handle, can shot, passes okay, and plays tough D.
That being said, when he's out, which is often, the Celtics tend to play Rondo into the ground. Arroyo would be able to give them 10-15 minutes of desent point guard play so Rondo will be have something left come playoff time.
I don't see any real down side to this signing.