Smart's absolute floor is someone like Tony Allen. I say absolute floor, because I would contend that right now at this moment Marcus is as good a perimeter defender as TA is, as a rookie. His shot is better than TA's ever was, and it's improving. His court vision, rebounding ability, ball handling skills and strength is better than Allen. Smart is a defensive savant. In 5 years he might be the best back-court defender in the NBA. That means more than you guys think.
His true position might be as a secondary ball handler at SG next to a scoring PG, ala Isaiah Thomas, whom I think he needs to play next to more. But we haven't even taken into account his infectious hustle, determination and leadership. Personally, I think he's one of the main reasons this Celtics team has overachieved so much this season. You just can't teach those kinds of intangibles.
I see why some people might be disappointed with his statistical output on the offensive end. But he simply doesn't have much opportunity to score. Even now where he's getting almost 30mpg, he's not really our primary ball handler because IT and Turner initiate the offense as much as he does. Stevens is primarily having Smart focus on two things: Playing amazing defense and improving his jump shot/shot selection. Bottom line, he could stand to be more aggressive, but he's just not getting his number called that often on offense.