Wow...
What is it about vegetarianism and meat that produces such heated discussions? Why, if everyone was this concerned about what they ate, we wouldn't even know what obesity was!
Oh right, about the iron/protein/whatever other vitamins/minerals discussion...
Iron: Anise seeds contain more iron per gram then any piece of meat. The only things that get close are intestines (Liver/Lungs/Spleen). Flesh (Steaks, chicken wings, etc) isn't rich in iron at all. There's a whole host of veggies/seeds/nuts that have over double the iron per gram.
Protein: The amount of protein varies greatly for every type of meat depending on it's source. Most common types of meat don't even contain that much protein. Often, there's more fat then protein in the meat. And although many people like to repeat myths about amino acids, veggies/seeds/nuts all have an amount of every amino acid. In fact, pumpkin seeds have more complete proteins then most meat types. (I think the only few exceptions are salmon and tuna)
So the only thing that can come close to proving the statement that being a vegetarian is unhealthy are the health risks associated with a B-12 shortage. I believe it can cause some serious damage to the nervous system since B-12 is used for the nervous system. So...for 1 vitamin, I'd say it's not a big problem to use supplements.
And about the initial question...
Being a vegetarian isn't really foolish, it's the motivation that can make it foolish. Predators have existed since...well...since life did. Only in the form of bacteria and other single celled organisms. But if we take the safe way and take a look at the time of dinosaurs, we can at least state with 100% accuracy that predators existed back then. Now that's already been millions of years ago, seems kind of silly why it would stop now with the introduction of "ethics". So the consumption of animals really isn't the problem, as it has existed for quite a long time without any problems. And as others have already pointed out, removing the predators actually has a much more horrific outcome. The only reason that I would see as "rational" for being a vegetarian is the one I have, bio-industry. Actually, I'm not really a vegetarian, I still eat non-cultivated meat. There's nothing really wrong with eating meat itself, it's the way we put the meat on our plate that's the problem. It's not even worth discussing wheter it's bad how animals are abused by people for profits and science. Everyone has his own view on wheter this is ethical or not, all I can say is: Don't complain when the same thing happens, but is put in a different light. Because when it's baby seals getting clubbed to death, or elephants being killed for their tusks, then it's suddenly "horrible" and "disgusting" while the very same thing is happening 24/7 in the bio-industry and labs with animals that haven't been living a happy life in the wild like the elephants and seals. And forcing vegetarian eating habits on predators is what I call "animal abuse". And yes, I know dogs can actually prefer vegetarian food over meat, I have 4 of them myself. But it's still animal abuse because dogs simply don't have the ability to live without meat.