Too many diet experts extrapolate from very, very small (tiny) population samples.
For example, if you exercise more than twenty hours a week, you will lose weight much differently than a normal person does.
Then there are the people whose set points are loosely held and who don't regulate their weight well by just eating until they have cleaned their plates. They can just go "hey, I've gotten fat, I guess I'll eat less" and they'll lose weight.
Then there are the "walk a mile every day until you are thin" types who do not respond to needing more calories by eating more. How many fat walkers have you seen? Women's magazine editors talk about those types of articles in terms of "if I read another one I'll puke."
Most people do not fall into one of those narrow groups. On the other hand, almost all personal trainer types are the "work out more than twenty hour a week" types. Big surprise -- their experience and observations are useless for their clients, most of whom stay fat.
The other two groups may have some benefits, walking reduces tension and is good for keeping mental acuity (for real, real studies show that walking on a daily basis preserves mental function as you get older -- it just doesn't make you thin), but as for a diet, if it really worked we would all be walking.
But at least half of the diets and health books out there make bad extrapolations and result in silliness. Nothing more.
Some good links: