The problem with that is, if you wait until he gets into shape and proves he can stay clean, you won't be the only team that wants to sign him. There's a distinct possibility that some other team beats you to the punch. NOW is the time to sign him, strike while the iron is not, and then, if it doesn't workout, you can cut him at minimal cost.
As Lou said, its low risk, high reward.
Oh, I know. You have to take a chance. He is the exact reason for low-base contracts with incentives.