Patrick,
Somewhere above you wrote that you are not buying the fact that too little sand can stay wetter then the "right" amount of sand. It can and this is not opinion, it is scientific fact. All soils retain water against gravitational forces, some more then others. Sand, because of it's particle size retains less water, but based on particle size distribution, different sands have different properties. We use a water release curve to determine at what depth the sand will release the water. Basically it is the weight of the water that breaks free from the sand and then drains. If we don't have enough sand depth, we don't build up enough water weight, and the water holding capacity of the sand is stronger then the gravitational forces. The sand will still dry out in time just due to atmospheric conditions, but probably not fast enough to keep up with irrigation water applied. Thus, whether you buy into it or not, the depth of sand in the bunker does matter, just as it does in a green or fairway capping. Sometimes too little sand is worse then none at all.
If you want more details I can send you real world test results that show how much water is held, and released at different sand depths.