In the article Zach Lowe dropped about a week ago, he mentioned that Boston would prefer a S&T involving Kawhi for Smart and the Kings and Grizzlies picks.
If that is the case, then it would depend on whether the Spurs like Smart, how much and long does his contract run and whether it is actually both picks (which would probably end up being in the top 5 and the 10-14 range)
There is obviously a huge fit issue to consider, he'd be a bench player and there isn't a rotation where he wouldn't be sharing the floor with another player that can't shoot (Murray, Parker, Anderson,) not to mention Spurs already have Parker, Murray, Green, White, Walker and Mills at guard with Manu potentially coming back.