Some of these grains/nuts/seeds are very easy to harvest in the wild compared to something as difficult as wheat. It seems natural to store them for the winter, and create various meals out of them. "Breads" and "polentas" are natural results of this.
And Mark Sisson seems to agree with me on both of these points. I believe he's said that you *could* ferment your grains/legumes/nuts for better eating, but under the framework of our society's supermarket, you don't have to. He's also said that cheat days are fine.
I don't call them "cheat days" because that implies I'm in a 'best nutriton ever' competition that leads to nothing but ridiculous nutritional perfectionism. Because of this, I happily avoid breads and the like because I WANT to avoid them, not so I can feel happy about myself knowing I didn't "cheat".