Pat, you're right that golfers can't always get well-imparted spin. But from a good lie in the sand, it's much easier for most players to spin the ball a bit than from other lies around the green. Assuming a level lie and a level landing area, a bunker shot is pretty easy to stop within 10 feet or so of landing for anyone using remotely sound technique, including me.
But I think you're hitting the answer with the last part of your post. I'm much more comfortable hitting the correct shot (a high blast with a bit of spin) from a below-grade lie than from an above-grade one. It's almost like the bunker face on the former makes it easier to commit to the shot required, even though the shot should theoretically be easier from the above-grade bunker (I'm not forgetting anything. The acceleration of the falling object you mention is exactly what creates the vector that should make it easier to stop the above-grade bunker shot, hypothetically, though I agree that the result usually doesn't support that hypothesis).
And as you, David, and I have mentioned, the bigger factor is probably the slopes involved. Above grade bunkers tend to give downhill lies to a target that runs away from you.