Welcome to a world of nutrition...warts and all!

I'm not, contrary to popular opinion, anti-vegetarian; I'm not even anti-vegan: I just want people to know the facts. If one refuses to eat meat (or dairy or eggs) for reasons they view as ethical (one shouldn't eat anything with a face, for example) then it's none of my business. In fact, in some ways I can see their point. But to adopt vegetarianism or veganism for alleged nutritional reasons flies in the face of all we currently know about nutrition.
I say "currently" because nutrition is an evolving science and, as such, we learn more every day. A former professor of mine used to preface every lecture with the statement that what she was about to teach us that day might be proven incorrect tomorrow. A stretch perhaps, but we got her point.
Certainly the notion that all of the essential amino acids had to be present in a given meal has been shown to be somewhat off the mark; if those amino acids are supplied on a daily basis, then the indiviual most likely won't suffer from protein deficiency. Minerals, on the other hand, do need to be supplied in the diet...and that's where a vegetarian or vegan diet can lead to problems.
Iron, calcium, zinc, to name a few, will be lacking in diets that eschew all animal products. Vitamin B 12 will be lacking in the diet of the strict vegan, and riboflavin will be in short supply. These deficiencies can be counteracted by adding supplements of course, but it strikes me that there's something not altogether "natural" about a diet whose healthfulness depends on pills.