I think being extremely overweight AND extremely underweight are equally atrocious. The ideal is to be at a healthy, natural weight for your individual body and its needs. I'm not sure there is a number associated with health - what is a good weight for one person at a given height may differ from the needs of another person at the exact same height. I think the question is more why are people doing this to themselves? Why did the eating disorder start? Why did the person continue in it? Where there resources available to them that could be improved to help them heal? How does anyone heal in a weight-obsessed culture?