Honestly, this is the entire hangup. If you don't understand the specifics of Martin's game, then you don't really have an argument either way. You're only arguing generalities like that Martin will be better if he takes fewer shots or has someone give him good looks. And that's pretty much assuming the system would make him a good player. You can keep denying it and then just restating it in different words, but it's true.
Martin has mechanical issues that limit his effectiveness on offense. When he has the ball in his hands, he's able to compensate by getting his own shots, getting to the foul line, etc. But spotting up, he has trouble because there are certain spots on the floor where he literally can't get off his shot consistently. One of those spots is the corner, and it's miserable for a Spurs' player to not be able to slide to that corner three. That is how he's like Jimmer. Difference between them as shooters is that Martin's release is faster but also off to the side. Both are low in terms of where they hold the ball. So Jimmer has a crazy-low percentage of assisted threes. Martin is not nearly as bad, but he also has a whole side of the floor where he's a below-average shooter.
When you say, "Martin will be better when he can focus on spotting up," it's kinda like saying, "Danny will be better when he can focus on driving." It's not using him to his strengths. He's only a good spot-up guy from the left wing (and right wing every other year for some reason). So he'd be the guy entering the ball into the post. But his release is still slow, so rather than having Green or Mills use their gravity to discourage help on the post-up, you have Martin's much lower gravity. And because of that release, he'll find himself run off three-point line, which is where his two-point percentage and below average A:TO come in.
This just like Jimmer. People just assumed that having a high percentage meant he was a good spot-up guy. And he would fit the Spurs because, , the Spurs have spot-up guys, right? It's not that simple.