Mike,
I agree with your visual contrast theory. Something stirs in us when we see woods or water, a sunset or fall color. Or a bear as we round the curve in a trail. Unless the light is low, turf grass lacks that stirring action. It is because of this capacity to stir emotion that the bunker has, as you say, "become an art form and maybe the most distinctive art form in determining an architect's style as he evolves".
As a bonus, sand provides a different texture to play from, adding an element to shot making, adding interest to the playing of the game. Simple crater type structures may be enough to defend greens (providing even greater defence than sand), but throw sand in the odd one and I think the architect has done something more than simply put up a defence. He's introduced a different type of shot. And a different type of look. I used to see sense in those who rail against course set ups where if you're good enough the smart play is to aim for sand rather than other grassy areas surrounding the green to increase the odds of getting up and down. No more. Sand ought to be different than turf but there is no reason to outlaw circumstances where turf is tougher than sand.
Should the bunker carry so much weight? That is a question I have often asked myself. But I have concluded that trying to argue it shouldn't matter is like trying to argue that an ocean or mountain or valley or stream shouldn't matter. They do. The trick is for the architect to not over-do the bunker and the art may be in finding just the right look and number, location and balance, both from a visual and playing context as well as with maintenance in mind.