There's obviously a difference between being (extremely/unhealthily) thin because you're
striving for it, and because you have real trouble with your metabolism etc.
Some people (@Amorous Eyes IIRC) on here are very thin simply because they can't gain weight properly. That's just a much a problem for those people as the opposite can be for some people (eating and exercising a normal and healthy amount, still being overweight and in bad shape).
People
promoting an unhealthy lifestyle (purging, fasting, waterbinging,...) to try and look like a skeleton is the exact same thing, and equally wrong, as people who'd go about promoting eating McDonalds 3x a day, drinking 10 gallons of coke and not moving, as a way of looking as much as possible like a blimp. Our current culture has a beauty ideal that lies closer to "too thin" than to "too fat" so this may seem strange, but it'd be just as much a possibility.
Everyone should try to be somewhat healthy in their food intake, and try to get some exercise. Your body should register OK in all categories. But as long as you're not too heavy and causing strain on your knees, or too thin and causing irregularities in your period, or too out-of-shape to move around your house, or have cholesterol that's wayyyyy too high,...you're probably in the "healthy" zone. And "healthy", folks, is really the most beautiful of all.
Does that mean it's wrong to want to lose a few pounds? No, of course not. Wanting to lose some weight, or gain some musculature, or just generally feel more active/have more energy, are perfectly good reasons to want to work on your body. Going to some extreme or another? Eggghhh.