My biggest problem is that there's a very realistic chance that he'll be injured just a couple of games into the season. He's been having problems with his legs, feet and all the concussions for quite a while now.
Second problem is that your 3rd spot on the team is taken by a veteran on his last legs. I'd rather see us getting a rookie to occupy that spot. You have a pretty decent hit chance when you draft a RB in a later round. Try to get a speedster to compliment Grant. Hope that Joe KcKnight drops to you, maybe move around the board a little.
Ted Thompson has proven me wrong before, though. If he does pass his physical and comes cheap, we might as well take a shot. At least it's better than having Ahman on the roster.