Believe me, I know. The state revenue "service" wrote me a letter claiming that there was no way I could subsist on an income as low as reported on my tax filing, and they demanded an explanation. Note that I mentioned red beans and rice in my post.
The disadvantage is, however, that starches break down quickly, and fats are more efficient for staving off hunger pangs. Plus, high-starch diets lead to insulin resistance.
I believe this is incorrect, as most of the world's population would suffer if this were correct. Historically and today in third world countries, most people subsist pretty exclusively on grain (rice, wheat, corn) and legumes.
I believe insulin resistance is more likely to be caused by high consumption of processed carbohydrates or sugars, not by grains and beans. Until quite recently in human history sugar and other processed carbohydrates, notably corn syrup, were not available, certainly not in sufficient quantity to pose a health problem. In addition, they were expensive and thus unavailable to the poor.