Why does every bloody thread on NS about fat girls or thin girls end up in silly debate with the worst arguments and so-called facts?
Problems associated with overweight:
Those who are have a much higher risk of developing some diseases. Diabetes, high blood pressure, osteoarthritis, strokes and cancer are the prominent ones. Other conditions are possible as well, such as increased cholesterol, sleep loss, gallstones, weak hip muscles and general discomfort. Constant fatigue due to the excess weight and combined with the lack of exercise comes alack of endurance.
(There is more stuff but this is from the top of my hat.)
Problems associated with underweight:
If you are underweight you can still develop diabetes in the case of rapid weight loss, since rapid weight loss affects insulin production. Second, you are more susceptible to illness, since you lack muscle and fat to retain body heat. Third, your body has little fat or muscle to use as energy, so poor endurance ensures, as well as constant fatigue.
Neither is a good thing, underweight or overweight. You can consider underweight less worse because most problems associated with underweight disappear if  you'd go back to a "healthy" weight. Also the gap from underweight to health is usually quite a bit smaller.
About "healthy weight" there is no true healthy weight everyone is different, but there is a general area in which someone's weight is considered "normal" and this is way better then the two ends of the spectrum.
Now about calling people "too thin"
You can't judge from a monitor, you can only make a slight assumption.
"Her ribs/hip bones are showing, she has anorexia!"
This is total bullshit, I've seen people with a "healthy weight" (even slightly on the chubby side) that had them showing, and underweight girls who barely had them showing.
Your bone structure has nothing to do with fat/thin, some people have more pronounced ribs/hips.
Also your length comes in the equation, tall girls on average are more likely to look skinnier at the same fat/muscle percentage due to their stature.
Now about Victoria secret models:
I'd safely say none of them is underweight (from what i've seen)