A good friend of ours has a cat that was chewing its fur off and required a cat food that is only available by prescription and she wasn't able to get it where she lived. Our vet recommended something similar and wrote us a prescription so we could get the cat food for our friend. After about 7-10 days there was a noticeable improvement in her cat's fur and about 6 months later her fur is fully grown back. She has the cat on 1/2 prescription food now and it seems to be working.
As with anything else, one should be wary of vets that are treating illnesses that are not visible and cost money to treat. As with human doctors, it's also wise to get a second opinion on these things.