I know that HNers will scoff at the citing of popular literature, but I find the arguments in "Diet for a Small Planet" and "In Defense of Food" quite compelling when it comes to "traditional" foods. Traditional diets are developed to make best use of the ingredients available in a region and are often the healthier than "scientific" diets. A good example is Mexican food. Corn and beans together make a complete protein.

I think that tradition, like cuisines, is an ever changing thing, however.