At the outset, I'd like to note that I spent several years following a vegetarian, then vegan, diet. I understand many of the reasons people stop eating animal-based food -- many of them are compelling and valid under the current mainstream meat eating/producing industry. However many of the negatives associated with mass-produced meat can be avoided by carefully choosing the foods we eat. The type of meat eating that I am defending is the occasional consumption of animals that have been raised on pasture in limited numbers and where the practice mimics that of nature and treats the animals (and slaughters them) humanely. In this limited capacity, I think meat eating can be MORE responsible than vegetarianism for reasons that impact the environment, our health, culture, history and morality.
Environment
Contrary to what many people believe, if done correctly, grazing is good for the soil. It also increases the presence of native plants, extends the growing season of grass and turns a resource that humans can't eat into a source of food. It is true that cattle do give off their own greenhouse gasses (but so do all animals!), however through proper management of cattle (that absorbs carbon), there can be a net positive in the fight against global warming (see various sources). All farming causes some impact on the environment, but the type of farming I am advocating creates a balance (see Alan Savory, Joel Sallatin, or J Bar L, where I shot this video). Finally, feeding cattle on grass requires zero input (from fertilizers, etc.) and lots of output. Grass-fed animals don't use soy or corn fields and often graze on land that wouldn't be suitable for vegetables.
We also have to consider wild animals. Humans have disturbed the earth in many ways, making it a challenging place for wild animals to thrive. An important part of making sure that the species that exist continue to live is managing numbers of that species as well as any invasive species that may be threatening the survival of other animals or plants. Thus, it is often necessary to kill some animals to ensure the survival of others (vegetarians debate this in regards to deer, but with invasive species there are a lack of other solutions besides hunting). In order not to waste, it makes sense to eat those animals.
Economy and Place
In places like Minnesota where long winters present a limited growing season, meat is often the only food that doesn't have to be transported across the country. The grass-fed model requires more human input and so is good for jobs and also gives farmers an opportunity to sell their product at a price that makes it more sustainable to stay farming. Pasture-raised animals also develop local economies as these farmers tend to sell within their community and support small local slaughterhouses (often a challenge, White Oak Pastures presents a good example).
Health
The health benefits of grass-fed meat are substantial and well documented. The main benefits are high levels of Omega 3 fatty acids, beta carotene and Vitamin E while being low in saturated fat. The health risks associated with meat are related to over eating of conventional meat, not responsible consumption of antioxidant rich grass-fed meat. Although eating vegetarian can be healthy, according to a report by the U.N.'s FAO and Standford University: "Animal source foods ... play an important role in ensuring optimal health and function, and their consumption is particularly important for women of reproductive age, fetuses, and young children"
Culture, History and Morality
Humans have always eaten meat. Without it, we would have ceased to exist a long time ago. We have teeth that are clearly designed to chew flesh. I question getting rid of something that humans were very clearly born to do even though it is no longer necessary for our survival. Meat eating also brings a great deal of pleasure. Prosciutto and sausage were created for survival but have became so much more: they have become part of our culture. These artisinal methods add value to our social fabric and should be preserved.
Is it okay to kill an animal for the benefit of humanity? I think that there is a cycle of life and that animals kill other animals and we are part of that relationship. These animals would not exist without farming and they can be provided good lives on farms. I believe that it is morally permissible to kill an animal that has been raised well and killed without suffering and then used (every part) to benefit humanity.
If you are thinking about becoming a vegetarian, don't stop. The world could certainly use less meat consumption. But if you are dedicated to responsible meat eating, then I say go for it -- there is a Portlandia episode made for you and me.
For more information on the benefits of pasture-raised meat, I suggest reading from two famous vegetarians: John Robbins and Nicolette Niman.