Back on topic I agree that Smart's offense particularly shooting and shot selection is a huge concern. He's certainly not shown anything close to being a useful offensive player in the traditional sense.
Now that said, there are players who find roles over time, with good coaching and serious work on their games. I think we have the first thing, and whether Smart puts in the time to become a better offensive player (and whether it pays off) is to be determined.
A few other points to consider though.
First, if you look at the C's offense over his first two years (which add up to one NBA season, essentially), it's better with him on the court than off:
105.7 Ortg on-court vs. 103.3 off.
The interesting thing is that this doesn't come from shooting - the C's shoot about the same in Efg% with him on vs. off.
What is comes from is that he is very good at helping is team generate possessions, in two ways one of which I think is overlooked:
1. Steals (and to a lesser extent blocks)
2. Not turning the ball over on offense.
The first is well-known, by the numbers the C's get about an extra steal a game when Marcus is on the floor.
The second is more dramatic - Smart himself rarely turns the ball over and the Celts turn it over on 13.3% of possessions with him out there, vs. 15.5 with him off the court.
The numbers are even more stark this year in a small sample, and what it means is that despite Smart's limitations on offense in terms of scoring, the Celts can often be equal or better offensively with him on the floor because they generate more possessions.
I think this is something that adds to his value in a measurable way...possessions help a team score. To me it means that Smart doesn't need to reach the same level in terms of standard offensive metrics (like shooting percentages) in order to be a "plus" player in terms of helping the TEAM score.