Fair points, though you're leaving out free throw shooting; Marcus was much better at getting to the line than Avery, and shot grade-A from there (one reason they posted him up so much)
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Defending a guy that shoots 35% from the field and 28% from the three can't be easy.\
And yet he gets to the line - a lot - anyway.
He shot a free throw for almost exactly every three FG attempts last season (.334, an excellent number; compare to Avery at .120) - and he doesn't use a whole lot of possessions.
Most casual fans aren't including FT shooting when they talk about "shooting", but it's a good way to score points, and Marcus, in addition to taking a lot of trips there, is money when he does: .812 last season, which is grade-A.
That helps the team get in the penalty, so that Isaiah and Jaylen can shoot more free ones, too. Casual fans focus on shooting and scoring, and are less aware of how a player impacts the whole offense of the team.
If he's going to continue to facilitate out of post-ups, he simply has to get better at putting the ball in the basket. Casual fans are more focused on his 3-pt shooting, but he was dismal at the rim: .488 is godawful.
The ability to get to the FT line and the way it enhances the efficiency/value of a player's scoring attempts is captured in TS% (which is called "True Shooting" but really is a measure of _scoring_ efficiency, not shooting efficiency).
TS% incorporates points scored by both FTAs as well as FGAs. So whether a player is efficient because he scores a lot on 3PT shots or because he gets to the line a lot, it incorporates both, appropriately weighted. League average TS this last year was about .550.
Avery's TS% last season was .548, or right about league-average.
Marcus' TS% last season was .486, which is beyond awful.
I mean, of the 174 players with at least 500 FGA, only 6 players had a lower scoring efficiency than Marcus Smart. (FWIW, Isaiah had the 9th
highest scoring efficiency in the league among that set)
Look, I like Marcus and I want him to succeed as much as any Celtic fan. And I don't want to give up and say their is no hope for his offense.
But there is really no point to arguments that try to waive off how bad his scoring offense has been. It has frankly, been truly, truly horrendous and denial of that reality is silly.