First of all, how is it that one doctor's opinion is considered proof that "diet drugs work"? I think the author needs to re-evaluate their understanding of scientific method and the criteria by which they assert facts.
Even if FDA has been given enough data to pass it's standards with four new drugs, this doesn't translate into an unfortunate situation where diet drugs "work" but aren't being prescribed.
Endemic to the problem of a diet drug is the issue where the patient seeks to find a magical way to eat bad things in a world where people can easily eat bad things all the time and are encouraged to do so. Alternatively the patient seeks to remove the urge. So a drug either alters the metabolism or suppresses the appetite. I've no doubt these new drugs make advances over their harsher or more deadly predecessors, but taking them fails to address the root of the problem.
Physicians understand this. When you go in to your doctor looking for a magical prescription for something like this, they ask you a few questions as they are required to do, urge you to consider that merely changing your lifestyle is actual solution to the problem, then resignedly give you the magic pill. The drug is supposed to be the last resort, but everyone from the manufacturer to the patient conspire to use it as a blanket solution.
And then you may get the effect you desire, and you also get the expense and side effects of the drug, and you've ultimately done yourself a disservice that will descrease your quality of life or possibly even kill you eventually.
Every doctor will tell every patient that they need to eat a better variety of foods, less unhealthy foods, and smaller portions. But then society and in most cases, the individual, proceed to ignore the true solution and encourage the opposite.