I don't know if its a sound argument to use the fact that we use grains to fatten our cattle up to not eat them as humans. I'm not saying I don't agree- but that's like vegans using the argument that gorillas are vegetarians so we should be too (in fact that's even better since we are more closely related to them). I think grains were important for human advancement because as a race we used to expend a lot more calories a day doing manual labor stuff. Now that we are so sedentary, they've obviously become part of the problem. So we either go back to busting our asses in the fields everyday or eat smarter I guess.
It's not a solid example I would use either, it's just interesting - Kind of like how it's supposed to be well known amongst pig farmers that they fatten better on low fat milk vs whole milk. However there's plenty of other reasons not to eat grains if you believe various sources.
I think with cows, the common theme is that grass is high in omega 3. Where as grains are high in omega 6. It's thought by some that this ratio of omega 3:6 changing so much in favour of Omega 6 is one reason for our (humans) increase in obesity (amongst other things). The same has happend with cattle (more grain and therefore more omega 6 and, they fatten up quicker).
I'm going to start sounding like a strict Paleo follower here, and i'm not. I eat grains and i'm not convinced that they're as bad as made out to be by some. There's a lot of very credible 'experts' who pick apart the basis for Paleo principles. I'm deffinitly not a 'paleo' guy but I agree with many of the principles. I'm very much against high carb diets, too, and I don't fear fat. Infact I love it. Saturated Fat in particular is the business, it makes you feel great.
My main disagreement is grains being the basis/foundation of your diet. Infact, I believe that if you turned the food pyramid upside down, completely reversing the order of it, so that grains were at the top, then that would be a good start and we would be on the road to some sound diet recommendations!
EDIT: Having wrote that post I had a quit glance at the food pyramid. Completely forget they put 'sweets' in with fats and oils. Guess turning it upside down wouldn't be a great idea after all but, i'm sure you get the point...