I have no heartburn about trading Marcus Smart whatsoever. Not offensively skilled enough to worry about trading. We'll be fine defensively.
Stevens wants guys that can switch in pick n rolls with out giving up much of an advantage. He hasn't had that ability to do that yet because of players like Sully and KO that can't guard anything faster than a sloth.
Smarts been able to guard 1-5 with some success. While AB can shoot better than Smart he can only really guard 1's and 2's. And IT, 1's. Smart has done a lot on the defensive end that makes up for his lack off offense. If he can become an average offensive player, which wouldn't take too much, he has a slight advantage over AB.
And I'm not saying Smart should be a starter, I don't think AB is really a starter, not on a contender. Butler fills that roll much more, but I'd rather have Smart and AB backing up IT and a Butler than get rid of one of them.
Unfortunately AB has the most value when it comes to D and the ability to shoot the 3 ball under that contract. If you're swapping one for Butler, AB would be the one that goes because you have to give up one less asset to make it work in a trade. And if you keep AB, he's going to demand a huge pay raise in a could years.
Smart will be more cost effective and you don't give up much.