[Sorry for the novella, but nutrition and human biology is something that's incredibly fascinating to me. The TL;DR is that nutrition isn't the same for everyone. We're biologically adapted to eat just about anything and you should eat whatever (in regards to a meat or vegetarian diet) makes you feel the best without negatively impacting your health.]
So much misinformation!
Before I go into the nutrition thing, I'd just like to point out that it's a commonly believed myth that hunter-gatherer societies had to work all day every day collecting food to survive. Time spent acquiring resources was somewhere around 15 to 20 hours a week. The rest of the time was spent in preparation of food or leisure. All said, hunter-gatherer societies has more leisure time than those of us living in modern societies. If you're interested, you should do a little research on the original affluent societies. Observation of current hunter-gatherer groups have given us even further insight into the lives of the original hunter-gatherers. It's pretty fascinating stuff. We pay a heavy price for all of modern amenities and our massive populations.
Anyway, nutrition isn't a "one-size fits all" thing. While everyone has the same general nutritional requirements, there are always going to be exceptions and special cases. The human body needs fat, protein, and carbohydrates. It doesn't really matter where it comes from (i.e., meat or vegetable), as long as you ingest the food, your body is capable of digesting the food, and you're able to metabolize the food. As long as you're not stuffing your face full of highly processed foods, anything you eat that meets the body's requirements for functioning is good.
From a purely biological standpoint, human beings are somewhere in the middle of herbivores and carnivores. We're omnivores, and like all omnivores we've adapted and evolved to be able to thrive on whatever our most common food sources are. In the case of humans, it's pretty much anything we can catch or find. We have a pretty unique mouth, for instance, compared to other omnivores who have very specific diets. We definitely don't have the same type of mouth as carnivores or true herbivores. Our digestive systems are completely different from either carnivores or herbivores. We have long digestive tracts compared to carnivores and we can't eat raw or rotten meat without getting very sick or dying, so we're clearly not carnivores. We have shorter digestive tracts compared to herbivores and we're incapable of properly metabolizing lots of raw, fibrous plant material, so we're clearly not true herbivores. Humans only developed the ability to digest animal milk fairly recently (around 10,000 to 15,000 years ago and in very specific areas) and in some parts of the world, the instance of lactose intolerance is above 90%, due to low genetic drift and a lack of animal domestication for dairy.
There is so much misinformation out there, being spoon fed to Americans, that we'll believe pretty much anything the media tells us at this point. Think about all the dietary fads over the last 20 years. Eggs used to be terrible. Then they were okay. Then they were bad for us again. Now they're good for us. That happens over and over and over again with a myriad of foods.
Moderation is not the answer for everything. There are certain things that you absolutely need to avoid. Added sugar (not the pectins from fruit or the sugars in breast milk, for instance) is something that should be avoided. Processed sugar serves no real nutritional purpose. The benefits that come from the quick energy boost are negligible in comparison to the drawbacks. Can you eat added sugar? Yes. Is it better for you in moderation? Yes. It is never, however, good for you. Added sugar is always bad for you (with certain, very specific exceptions). Even foods that don't contain added sugar can be bad for you, depending on how they cause your blood sugar to respond, because the body actually creates sugar when metabolizing certain foods. It's part of the reason processed foods are so insidious.
Nutrition isn't as easy as just reading a label. There is an entire science devoted to understanding how and why the human body processes foods the way it does. And it's constantly changing. Part of that change is dictated by actual science and part of that change is dictated by money. Big corporations want you to believe that added sugar is fine so they can sell you Malted Frosted Sugar Bomb cereal.
Whether you should or shouldn't eat meat depends on a lot of things. We have the luxury of making that decision because of the society we live in. In other parts of the world, it's not a choice. You either eat meat because it's available, or you don't eat meat, because it's scarce.