Firstly, Ainge has already decided. He has a value for Smart and if someone else exceeds it? We lose him.
Smart is not about offense. That is an understatement. Due to his style of play, he can make some amazing offensive plays when acting as a facilitator. Defensively, he is extraordinary in a league that insists on offensive competence.
Smart calls the execution when he is on the court. He plays harder than anyone else, thus, he is the team's leader of a form.
He wants it more than anyone else and expresses this through his decisions. We can all agree with 2:00 remaining in a tie game, we want Smart on the floor first, then, the rest of the team. The opposing coach has to match Smart's defensive commitment. He has to include the Celtic's "back up point guard" in his offensive play calls and the opposing players know this.
I doubt Ainge will go into luxury tax to sign Marcus Smart, but, if he does? We know who will be calling the defense on the floor in game six with 1:59 to go in a tie game against Golden State won't we?