Pet diets - Variety unacceptable?

The other day I read on my favorite topic “How to keep my pet healthy” in one of the news feeds again: Feeding your pet a variety of foods can minimize the development of food allergies. By providing pets with different proteins and forms, like kibble, canned and raw frozen, a rotation diet offers the full spectrum of nutrients cats and dogs need to avoid food-related health problems.” It seems as if rotation diets are becoming more and more substantial part of “health diet” advice. When I read and hear about it a little 2 picture graphic, which I saw in one of the publications I was reading comes to my mind. The story is about Mrs. Jones. In the first picture, Mrs. Jones is shown with her puppy in her arms and her veterinarian handing her a bag of dog food saying: “Be sure to only feed your dog this 100% complete, scientifically balanced food every meal for the rest of his life” To which Mrs. Jones responds: ”Isn’t science wonderful? It’s such a relief to know that I can feed just this one food and my pet will have the very best health.”
The next scenario shows Mrs. Jones, with her baby in her arm visiting her pediatrician who gives her word for word exactly the same advice she was given by her vet for her puppy earlier in the day. The only difference is that the pediatrician uses the word “child” instead of “dog” as her veterinarian did. And her response is now: “Is he nuts? No way! I am going to give my child variety and fresh foods.”
The author’s comments on the bottom were something like: “The same common sense people apply to themselves must be applied to our pets. Pet nutrition is not a special case situation requiring the intervention of food processors or nutritionists.” And later on he asks: “Are our pets, mammals like us, so physically different from us that they don’t have the same need for freshness and variety in their diets?”
But enough of the preaching already, everybody hopefully understood by now that I am pro rotation diet. What gets me concerned every day over and over again is the vast amount of my customers coming to me, telling me that they are feeding their pets the same food for years. That they tried a different food sample once and the pet got diarrhea. Well, as I always say: Let me feed you every day steak only and nothing else for let’s say a year. Then, all the sudden a new day comes around and I feed you veggies instead. You come up with your own conclusion. Just let me say this much: When making diet changes, make them gradually and slowly, give the pet’s systems time to adjust.
Then, by the above described customers I am quite often also told that they feed their pets like this because their vet told them. Well, my first suspicion is, which food is the vet recommending and maybe there are business incentives for him behind his advice. But that is stretching my fantasy and I don’t even want to go there. It certainly would explain a lot, however, just a couple days ago I had a woman, who just adopted 2 puppies and buys her food at a regular retail store was approached by store personnel to feed something different. She, worried as she is about her babies talked to her vet about it and he clearly told her: “Don’t do that, you are upsetting the dogs’ health.” Go figure…
What I would like to know is, what do you think about this issue and are you practicing varied feeding? Have you ever tried it, what were and are the results? And everything else you want to share with us.