I'm not gonna say Smart is an upgrade over Bradley. Both are plus defenders, Bradley is the better scorer for sure. But Marcus is a way better passer, ball handler and facilitator. Plus he gets into the paint much more than Bradley.
When paired with IT, being a solid facilitator, and having the ability into the paint is helpful. It will take pressure off of him, and allow him to get more open looks, without having to take the physical abuse of getting to the rim when all else fails.