Dr Steven Gundry has an interesting take on the issue with his book The Plant Paradox. He’s a cardiac surgeon, and gives his reasons (most of which are evolutionary) together with the microbiology. @Barg, nuts are an important part too.
Basically your gut is not adapted to grains (introduced only 10,000 years ago) or New World foods (only 500 years ago), potatoes, tomatoes, sweet corn, legumes, chilies etc.
