Corn is corn, and shouldn't be in a dog food, be it Science Diet, Iams, or Alpo.  If you're wanting corn to feed your dog, then get the grocery store brand, it's as good as this garbage, and you don't pay as much for it. 

As far as vets knowing nutrition, they know what they're paid to know when it comes to food.  If Iams helps them get through med school, they support Iams, likewise for Purina.  My vet told me to use common sense and feed what a dog would eat in the wild (he's 60 years old and doesn't owe anyone anything.)  He said flat out, do you ever see a dog in a cornfield???  That's why I pay a little more for real food with meat in it.  Big bonus, no huge piles in the yard since it isn't going in one end and out the other, but that's not something most people give any thought to.  If they use what they eat, they WON'T excrete it. 

As far as different formulas for different dogs, can a Dobie handle more corn than a poodle???  Do black humans need to eat different from white humans to be healthy???  If Royal Canin had their way, blacks and whites would both be eating different grades of artificial meat....