I've said this in a couple threads lately. I love Smart and don't want him to play for another team, but I think his on court impact is overall not as a great as fans might think. I know he makes winning plays, but I also think he makes a lot of losing plays.
Not just the bad threes--and there are a lot of them. He demands the ball on offense, and he's probably the fifth best offensive player on the team. He has been posting up a lot over the last few games and, honestly, not doing very well at it. He drives to the basket and takes bad shots. He's also pretty turnover prone.
I've also noticed that he is getting stuck on more screens so far this year. He isn't slipping them as well, which is leaving his man open for a lot of threes. He's a great defender against bigs and wings, but I just don't think he's a good defender against shooters or guards. He isn't quick enough to get around screens.
Smart's defensive playmaking is a luxury, but the cost is 3/14 with a ton of offensive stallouts because he is demanding the ball in the post or making bad drives.
He's a perfect fit if he's willing to take 4 shots, move the ball, and do the little things, but he has consistently proven that he is too willing to try to take over a game offensively. He's a fine offensive role player, but when he tries to take the role of offensive playmaker, he is one of the worst in the NBA statistically.
All of this being said, I don't think we need Smart to be a championship contender. Brown, Hayward, and Tatum are all really good defenders. Walker is a really good defender as well. His defense is a luxury we don't need, and it comes at the cost of 5-8 bad offensive possessions per game.