These plots in missouri are on very good ground, heavy silt loam. They have been limed 2 years ago, and have about 300lbs per acre of 17-17-17 applied each year. And no, yield very rarely will increase much from raising plant population over 150,000. I do it because I feel I will lose 100,000 plants due to deer browsing, putting me right where I need to be for normal harvest population, if I were to harvest.
Yes, more plants in light ground can help shade the ground, to help conserve moisture, but can also result in very short plants if it gets too dry for to long. Short plants are not good for bean production. Problem that can not controlled when farming sand without irrigation sometimes.