So what's the deal with grains? Grains are a part of our daily lives, and have been for 10,000 years. According to Loren Cordain, maize, wheat, barley, and rice account for 76% of the edible plants grown on our planet by gross tonnage and 56% of man's food energy. 56%! Grains are a cheap source of calories, and unfortunately the majority of our diet is made up of grains. They contain little nutritional value in their raw state, which means they have to be processed in order to be even slightly valuable as a source of energy. This fact by itself should steer us away from grains. Processed=Bad.
If you want a short anecdote about the badness of grains, Everyday Paleo recently wrote a concise explanation of the subject.
If you want an analysis backed by science, Loren Cordain is the way to go. Unfortunatley, I can't do her work justice. Take the time to read it.