From some stuff I've read, whether we crash off the weight or follow the solid 2 lbs/wk advice, most of us will put it back on within five years. Part of it is lack of planning or the mindset that diet is a temporary thing, but part of it is some hormone that makes people who lose a lot of weight feel hungry.
That said, while I find primal to be a very healthy way of eating, for those who need more than a few pounds lost per year to stay interested, I don't see any problem with occasionally doing a three day fast or doing 400-500 calories twice a week, or even eating a single food for a week. When one considers the amount of disease attributed to obesity, I believe that anything that doesn't harm a person is a good way to lose weight.
I know I always come back to quitting smoking, but I find that the same people who would be fine with someone taking an antidepressant to quit smoking are far more critical of people who want to lose weight taking short cuts. So, even people who are "into health," don't take the problems of obesity seriously. It's not a vanity thing. A person who smokes will probably die of something related to smoking. But it is also true that a morbidly obese person will also most likely die from something related to that obesity.
"Right is right, even if no one is doing it; wrong is wrong, even if everyone is doing it." - St. Augustine