I've been eating Paleo for 4-months and I've never felt better. For those that "couldn't do it" it's because they haven't tried it. After the initial transition off of the carbohydrate rollercoaster it was totally easy. It's not the traditional calorie-restricting diet. By eating your proteins and fats and not confusing your body with all nature of modern carbs your natural feeling of satiation returns. You won't feel hungry, you won't have cravings as you will feel full.
Lastly it need not be pointed out that 'Grass-Fed' beef requires NO GRAIN to raise as it doesn't eat it. Sounds like the nutritionist was just spouting the typical anti-omnivore dogma of the past few decades of pro-vegetarian thought.
Grass-fed beef is as healthy as bison and naturally high in beta-carotene (I'm not kidding) and omega-3. Beef raised on grass have no more ecological impact than bison on the prairie. I.E. None.
I have no choice--I'm allergic to grains, legumes, dairy, and most nuts, as well as a few fruits and vegetables. For me, it's either Paleo or die with a glass of water in my hand.