While there has been some contention about the completeness of proteins, this isn't exactly a great source. Everyone seems to have an agenda. His statement that it is impossible to have a calorically sufficient, protein-deficient diet tells me he's never heard of kwashiorkor or marasmus. It is entirely possible. Big red flag.

It should be fairly obvious to anyone familiar with animals (particularly ruminants) that we do not digest certain plant materials (like cellulose) as well as they do. The author makes the very wrong assumption that we do, and can extract whatever is in a plant. The truth is that we can't because there is simply so much other material, some of them in fact anti-nutrients (e.g. oxalic acid and tannins), associated with the amino acids. There is a system called biological value that calculates the amount of the protein incorporated into the human body when we eat a food. It is typically quite low for plant foods, and quite high for animal foods. This is one area where animal foods are actually beneficial. Others have mentioned B-12 and heme-form iron. We are omnivores, and can benefit from eating a wide variety of foods.

I do agree that the meat and dairy lobbies have so much clout in this country that they have clouded our notions of nutrition. We certainly don't need as much protein as we usually get. Check out Marion Nestle's Food Politics for an in-depth look. The chapter on the food pyramid alone is eye-opening.

The problem for those training, particularly with heavy weights, is that the protein-density of most plant foods is very low, and its absorption is slow. Even if it were possible to get your generally-accepted recommendation (in this case around 2g/kg of protein per day) from eating nothing but spinach, you'd get full first, and you'd have a hard time getting timing absorption immediately before and after a workout. Whey (or soy, to slightly lesser degrees) protein is useful because it has a high biological value, is protein-dense, and is easily digested by most.

For long-term survival, vegetarianism can be great. It makes eating for athletic performance a little more difficult.

In any case, OP, I actually like Clif bars. Lots of soy protein and carbs to keep you going. The fuel to keep going will be your primary concern, though you should include some protein. Protein isn't the problem most make it out to be, though it is worth considering for endurance athletes.