Obese people are really no different than someone who is 20 pounds overweight. Obese people, like anyone else, need to find the diet that works for them. Every fad weight loss diet works for some people. The key is not whether a diet is a fad, but rather how well an overweight person is able to adhere to a particular diet and develop self discipline. Once the pounds start coming off, it becomes a matter of pride and provides the incentive to change eating habits.
Obesity, IMO is a medical issue that requires a more comprehensive approach than a fad diet can offer. The obese person's medical health, state of mind, and nutritional habits must all be considered and treated simultaneously. Fad diets often offer some "catch" that promises a quick weight loss with little effort. Obesity must be treated over a period of time by a slow and steady course wherein weight is taken off gradually and permanently as overall physical and mental health is monitored and improved.
Fad diets aren't the answer to the obesity epidemic. I think the key to helping the obese are fundamental lifestyle changes. Nobody ever adheres to these fad diets, they are impractical because they are often overly harsh and unrealistic. Proper nutrition and adequate activity are better solutions for dealing with obesity.
I do not believe fad weight loss diets can help obese people. If anything, these hard to follow diets make it more difficult for them because they know they can't keep up with the diet. Obesity can be caused by a number of health conditions, some of which even doctors won't make a direct link to with the first few visits. Obesity is best cured with the help of a doctor, not a fad diet.