That's an honest vet! People who think the more expensive the dog food is the better it is are usually just making themselves feel good while the dog would do just as well on road kill. I'm sure I'll be considered an animal abuser but my dog has been on Walmart's Old Roy Complete dog food since I got her as a one year old and she turned 14 in September. She still bounces around like a five year old and hunted for me this fall. I may have fed something different if I hadn't been an animal nutritionist for many years and understood that the source of the nutrient isn't nearly as important as the correct balance of nutrients. Many of the expensive dog foods are overkill as far as nutrients and they pray on our idea that animals know the difference between a corn/soy/chicken meal feed and one with lamb/peas. Don't get me wrong, I'm not saying Old Roy is a top of the line dog food, I'm just saying it's plenty good for the majority of dogs.
No, you can feed 'em whatever you choose to. I can attest, though, that going up in brands was REALLY NOTICEABLE within a few weeks. Yeah, I pay $100 extra a year but what I see in return:
1) Less shedding, LOTS less
2) Less poop (more absorption)
3) Don't have to serve as much food (fillers require more food going in)
I did start with Purina One. Then I moved up to Purina Pro Plan. Now i'll stick with Chicken Soup or Canidae because there's zero human ingredients (chemicals, byproducts, or coloring) or filler products.
Some humans don't mind McDonalds every day of their life.... I just prefer not to.