I think the facts surrounding the issue of whether or not fat vs lean is healthiest has been greatly distorted.

For example, those who are trying to push a lean-is-better agenda will deceptively site research showing ill health for those who are extremely morbidly obese (bmi > 40) and extrapolate from this notions about moderate overweight and obesity. This is a hair away from lying.

Then on the other side the fat-is-better people take research showing the underweight have health problems, and then try to paint the situation that fat itself is somehow protective. However, the fact activists don't control for those who are underweight due to nutritional deficiencies and malnutrition (whether self or environmentally imposed) so the picture is skewed. Comparing someone who has anorexia and weighs 110 pounds, to someone who has a healthy diet and weighs 110 pounds is false.

I think both sides are lying but also telling the truth. The lean advocates are correct in their assumptions that the sort of environment which results in obesity and morbid obesity is not one which will maximize health. It implies metabolic sickness (IRS), and it implies one is purposely eating energy they don't need (which puts oxidative stress on the body and exacerbates aging). Extra fat itself can be deleterious to health too. The increased system stress carrying around more metabolically active tissue has is also of great concern.
However, they try to make it seem as if 65% of the population is overweight enough to suffer these problems when that is just deceptive. They take risks that the severely obese suffer and say it is a risk for the man or woman carrying a spare tire. It's not true. While the moderately overweight man or woman may or may not have an environment that is health-causing, moderate overweight itself is not a health risk. Severe overweight, however, is a health risk (fat is metabolically & hormonally active, therefore carrying around dozens of pounds puts stress on the body, accelerates aging, and exacerbates hormonal problems). People often purposely or naively confuse risk factors for obesity with obesity itself, and therefore diseases caused by the risk factors are too often blamed on the fat itself. Like I said, severe overweight is probably a health risk, but moderate overweight (i.e. about 90% of the "obese america") is not.

As for the fat advocates, they are correct in saying that trying to diet down to unnatural slimness has no protective benefits, and one does risk malnutrition and ill health. Forcing your body to do something it wasn't meant to do can't be the road to good health. We forget sometimes that the goal of the game isn't to look like kate moss, it's to be healthy. Healthy might be carrying a few extra pounds for some people, and for other people it might be being a bit more lean.
However, they are incorrect in their assumption that excess fat is just another physical trait like the hair on your head or the color of your eyes. This is a fallacious belief for numerous reasons. First of all, fat is metabolically active - it's alive, it burns energy, it requires energy, it gives hormones. Therefore, an overabundance of fat is both a symptom of disease and contributes to disease. Second of all, the state of any physical trait - metabolically active or not - is indicative of health. Weak nails and dull eyes signify deficiency. Gaunt thinness is a sign of disease (metabolic or psychological). Yes, extreme obesity is too sign of disease (metabolic or psychological).
Just the fact that so many more people are obese today than a few years ago is enough evidence to conclude that most of us who are obese are obese because of our lifestyle. No one is designed to be extremely fat to the point of compromised mobility. What purpose would that have from ane evolutionary standpoint? Those who are extremely overweight have some sort of unresolved ailment, and they should seek a cure. While I think it is admirable to fight for human rights, we should not proliferate lies that encourage people to maim themselves, or to "accept" ill health.

I think the fat activist movement should take an approach similar to activists who fight for handicapped rights. Handicapped advocates ask for equal treatment regardless of impairments, and this is what fat people need. Fat people don't need to be told "it's ok, you're fine the way you are... have another coke". Imagine how ridiculous it would be if handicapped advocates tried to stop the research which benefits those who they purport to defend -- all because by trying to "cure" handicapped people you imply they are somehow defective or less worthy. This is what fat advocates are doing today.

What really should be encouraged by the fat activist movement is divorcing self image from body image. Once this divorce has been made, one can seek to improve their health and body, without it affecting self worth. It's ironic really that so many fat advocates have yet to learn this. Instead of trying to say "I love myself and am a good person in spite of my obesity", they love themselves because of their obesity. The problem is these people are the only voice fat people have, and they are not working for our best interests. I want to know what are the causes of the disease I have called morbid obesity. I want it to be cured, because it is a terrible affliction.