I think the FA movement, like all movements, has people who believe extreme things or hold some ideas that are probably questionable, etc. But it also presents a necessary message that I don't think is probably getting out to most people...that weight doesn't equal health, and that it's great to take care of yourself however you see fit because you LIKE yourself, but don't need to because someone shames you or says that you SHOULD.
There's a lot of science that supports the things the FA movement says...not just about the rarity of achieving permanent weight loss, but on the health impacts or lack thereof of losing weight for the post obese.
For example, being "Obese" doesn't increase your overall risk of death at all (Morbid obesity and super-morbid obesity do increase your overall risk), and the BMIs associated with the lowest risk of all-cause mortality are in the "overweight" range. You're statistically more likely to die at a low-normal BMI than at an overweight one
http://jama.jamanetwork.com/article....icleid=1555137
In terms of morbidity (basically, overall disease state), overweight slightly increases the risk of some diseases, and decreases the risk of others. There's no overall disease impact of being overweight vs normal weight by BMI:
https://www.aerzteblatt.de/int/archive/article/66217
Any weight loss increases your overall mortality risk, unless you are diabetic or start with a BMI over 32. Losing more than 15% of your maximum weight is associated with increased risk of death regardless of how high that maximum weight is:
http://www.nature.com/ijo/journal/v3...jo201041a.html
http://iussp2009.princeton.edu/papers/92930
So it's certainly not a given that weight loss in and of itself will help you live longer, or not get sicker, especially if you're not in a BMI category over 35, and there's some evidence it'll hurt you.
There's also a growing body of research separating people who are active and eat healthy foods, but who are still overweight or obese...basically, trying to answer the chicken or egg question of whether it's the excess fat that is risky, or the lack of activity and diet.
So I believe FA is promoting a valuable conversation, even if their message isn't always perfect.