When I was in high school, I had an acquaintance of Indian extraction whose father was a doctor and always told his children that, quote, "food is better wasted on the plate than in the stomach."
I remember when I first heard this statement it was mind-blowing for me, who had been raised in a clear-your-plate home. The logic of the principle is immediately clear and undeniable: if you have eaten your fill, there is no value to clearing your plate as the food is wasted either way--better to waste it on the plate than waste it and overeat at the same time.
Still, I struggle with the desire to clear the plate, for reasons completely non-gustatory and non-economic, such as avoiding a conversation about the quality of the meal, or embarassment at taking more food than needed. But these are foolish reasons for overeating.