I think as soon as you see Smart as a role player on a contender, he's excellent. Where you run into issues is if you cast him as a starting PG, which may have been the expectation when he was drafted.
He's a high leverage player who makes your defense better, plays hard, shows leadership/ motivates other guys, and can win you some games on the biggest stage. One thing I think is frequently underrated is his playmaking. If he can pass and be a ball-handler at the backup PG level, plus everything else he does, I think he can be pretty valuable even if his shot remains the same. That gives him 4 starter level skills: defense, passing, ball handling, leadership.
I agree with folks who want to lock him down for 4 years at a "reasonable" cost for a 6th man. Under a $100M+ salary cap, a versatile 6th man in his prime years on a contender is probably worth a 4/$50M deal IMO, and you think about moving Rozier for a (cost-controlled) draft pick or player and rolling with Irving/ Brown/ Hayward/ Tatum/ Horford (Smart) as your core.