I'm hearing more and more of the narrative from sports media that Smart is probably the best 1-2 defender in the league.
On top of that, he is ridiculously versatile as a defender. He is the primary helpside defender often when he is on the court, drawing charges and getting helpside blocks. And, he does the "get out" thing when there is a mismatch that the other team is trying to exploit in the post, and he switches over from the weakside to take them instead.
On top of that, the Celtics are a really good offensive team when he is on the court. He is probably our best distributor out of the pick-and-roll (although Kyrie is figuring that out pretty quickly too, he is primarily a scorer out of the pick-and-roll).
My theory is that if you have a guy on your team that is the best in the league at something, you don't trade him for potential, or for an average player. A player with an elite skill is better than a player with different average skills.
And Smart's elite skills happen to be some of the most coveted (defensive versatility) and most rare (ability to defend on ball really well without fouling) in the NBA.