Smith would be excellent as a 3rd option. If you look at the #3 options around the league, you'll see how favorably he compares to them (and how ridiculous the notion of him being a 4th option is).
Bosh, Ginobli, Amar'e, Ibaka, Thompson, Conley, Hibbert/West, Boozer, etc. are the best 3rd options in the NBA. When defense is factored in, JS is on their level and is asking for a similar salary (except for Klay (who's still on a rookie contract), they're making over 11M to Max).
If the C's can get him without taking away from the nucleus of the team, it'd be an excellent addition. Green and Bass/Lee (possible 1st rounder) for Smith and that's a serious title contender. For Pierce? I'd have to strongly consider it.