Riiight. Okay, it's a tough one.
I think shake diets are good for rapid weight loss. They can be really useful to kick start weight loss, especially in patients who are extremely obese and trying to lose weight for surgery, or say patients with arthritis who would be able to exercise more with less pain if they lost some weight fast.
I think they are a short term solution.
Ultimately, people go back to eating food and unless they are following a strict maintenance plan with a healthy, balanced diet AND regular exercise, they will gain the weight back.
Shakes have NOT been proven to be effective in the long term, because people tend not to go into this "maintenance phase".
I think what happens, is people think it is easy - yeah, I just drink these shakes, eat a healthy dinner, and I lose heaps of weight! No need for exercise! And they do lose weight.
But once they go back to normal eating, their metabolism has slowed even more than before (because of the low calorie diet) and they begin to pile on the weight again.
People are also less likely to stick to shake diets in the first place, because they can be restrictive and therefore people will get hungry, frustrated, and then binge.
I think shake diets can be useful in certain circumstances, when supervised by a doctor/dietician/naturopath/chemist, and when a specific maintenance plan is put in place.
Generally, I would ALWAYS recommend a healthy, low fat, balanced diet, with NO skipping of meals (especially breakfast) and DAILY exercise. And for the idea to be that this is the same on maintenance, although you may be able to get away with eating a little more. This is HANDS DOWN the ONLY proven way to lose weight, and keep it off for good!
Nobody likes the concept of the "lifestyle change" because you lose weight slowly - sometimes only 1kg per month, and we live in a time when we are extremely impatient, and want what we want now. Also, I think people want to lose weight to look a certain way, but don't necessarily care about being healthy - only about being a size 8 or 10 or whatever. Until this mindset changes, fad diets, shake diets, diet pills etc will all remain popular.