Obviously it would be silly to call Marcus a shooter, but despite his ugly overall shooting numbers for this season, I wouldn't call him a non-shooter.
He shot a respectable percentage from deep in his first season and, without checking any numbers, he looks competent with the mid range pull up.
Most importantly, though, his jumper is not significantly broken looking. It looks pretty smooth.
I'm convinced he'll settle in as an at least 33% three point shooter in the NBA. He won't be an offensive super star, but his defensive play coupled with solid offensive play could make him a borderline star and leader on a very good team.