I think a big problem with this debate is that people view diets as an exclusively physiological thing. There are huge psychological components to these things. Why do we need diets in the first place? Because we have trouble exercising self-control. The most important criterion for a diet is whether it is psychologically compatible to the dieter to the point where he or she can generally follow it. Of course jogging and weight training every day you're on a diet will make the diet go better IF you can do that. More often than not, if following a diet is really onerous, people fall off it, feeling that they simply don't have the capacity to diet.
Now, I'm not a big fan of this cleansing thing but it's helping a guy lose weight. Why tell him to do something that's much harder that may have the net result of him not dieting at all. If you undermine his faith in his diet while simultaneously suggesting a course of action that is much more difficult, the net effect is actually to discourage him from dieting at all because you're making it "too hard."
Nobody decides to become fat. Obesity is a disease of the will not of the body. The discourse on this thread is increasinly moving towards an assumption of absolute control of the body through a perfect will. If this were the case, nobody would need to diet in the first place because nobody would have gotten fat.