I don't buy into the allergy angle. Nor do I buy into the nutrient deal either. They are all supplemented pretty much the same, the only differences between foods is going to be the quality of the ingredients that are used.
Not all dogs have allergies, or are sensitive to certain ingredients. If they are fed the same grain their whole life it's unlikely they will "develop" allergies to it. Allergy dogs are born with them and it takes a certain amount of time for the antigens to build to a point where they are symptomatic. It depends on the severity of the allergy as to how long it will take for the symptoms to show. Torque, my boxer, was 4 when hers appeared. Hers were fairly mild and were controlled with shots very well. Goliath was already a mess at 18 months and his are severe. The shots have done nothing for him. Rarely is it just food or just environmental/inhalents. They usually go hand in hand. I've run into dogs with severe environmentals and very mild food and the other way around, both of Goliaths are severe so it's a nightmare.
The rotation of food is to keep dogs from getting bored. Most of the small breeds get bored or dictate to mom and dad what they will and won't eat. Not all but a lot of them do. I like the idea of giving them different meat proteins too, with a food like Canidae that's hard to do. They have a lot meat proteins just not much of each kind. One of the things, the many things
, I like about the Timberwolf products is they have a good selection of formulas. I rotate through their food because of the quality of his ingredients. He has chichen, chicken/turkey, lamb/venison and fish formulas already available. Buffalo and a straight venison coming. Even allergy dogs have a good formula that they do well on long term. I like that a lot.
Sorry for the long post, it's early.