This article puts the case for dogs being carnivores and not omnivores. The reason that this is significant as a dog owner, is that the majority of manufactured dog food has very little meat in it because grain is a much cheaper and easier to handle food.
Now it is universally agreed that cats are carnivores including the domestic cat. This means that pet cats should only ever be fed meat. If they are not fed meat they will have a large difficulty in digesting and using the nutrients provided in the manufactured food.
So point one for dogs being carnivores might be that if your vet recommends that your dog have a diet on anything other than meat, how can they be trusted if they also sell cat kibble on their shelves? And why would you need to process, and preserve meat for a cat to eat anyway? Surely the healthiest food for cats would be raw meat diets, and a poor substitute would be all meat cans.
The major argument for dogs being most eaters is that their direct ancestor is the grey wolf. The wolf is a meat eater that hunts, kills and eats its own meat. If dogs are forced to, because of the lack of near by prey, they may eat a very small amount of berries, but they do not feast on fields of wheat, corn or rice.