One problem with using FG% as the primary judge of an offensive player's efficiency is that the circumstances under which each player shoots are much different. When defenses clamp down or the shot clock is running dry, someone on the team is going to have to find a way to get a difficult shot off. In general no one can be too happy about having a 40% shooter fire away, but if that same 40% shooter can be almost as effective even when the going gets tough, then he starts looking a little better.
That's one of the reasons that Marbury's indiscriminate three-point bombing becomes a more fearsome weapon late in the game -- if the team isn't all that likely to get a good shot off anyway, then someone who can shoot in the low 30s from 25+ feet, with no warning, really isn't that bad an option.
We've all seen Marion's FG% plummet at the ends of games, when more physical defense is permitted and everyone gets a bit tighter. It's great that he is able to pad his percentage by getting dunks and layups, but that doesn't mean he is a good shooter. And regarding his three-point percentage, my impression from watching games is that he shoots them much better from the corners than from elsewhere along the arc -- which is perfectly fine, as long as he sticks to the corners, but it doesn't really provide additional evidence of how good an overall shooter he is.
I didn't think much of Iverson until I started watching him more, starting with the 2001 (?) Finals. He attracts so much defensive attention that, even when he misses, his teammates have great opportunities to clean up the mess with easy buckets. I don't pretend that this is any great insight, but I think you really have to watch it for a while to appreciate how dramatic the effect is.
In "short" (heh), I think Iverson is a unique case. I don't consider him a top-ten player (er, at least probably not, I'd have to think about it), but I do think he brings more to a team than Marion does.
All that said, it's true that one should be careful not to overvalue the ability to create one's own shot. Marion's weakness in this regard means that he will never be a superstar, but I agree that he makes up for it in other ways. You could look at someone like Desmond Mason and say that he can create his own shot, but I'd much rather have Marion on the team than one or even two Desmond Masons.