I've always been mixed on that.
I respect Duncan's right to choose what he'll endorse and how he's marketed. But I think the League has missed a great opportunity to promote an elite athlete, who happens to be well decorated and "clean", as a role model. I don't know exactly how much Duncan controls and how much is the League's decision that maybe he's not marketable because he's "boring."
Either way, I think Duncan could have, and should have, been promoted much better than he has been. I think he'll be appreciated even more after he retires. Like the song says, "Don't it always seem to go that you don't know what you've got 'til it's gone."