Fad diets just end up making people fatter in the long run.
Extreme calorie deprivation does one thing long term: It teaches the body to pack on more fat for the next starvation cycle.
In fact, the fad dieting craze that started in the late 70s with the low fat diet is, IMO, one of the causes of the obesity epidemic today. People who would be naturally pudgy end up obese after starving themselves over and over again.
The main cause is, of course, a near total lack of activity. The explosion in obesity rates coincide with the rise in popularity of cable/sat TV, the internet, and home video games.