While there is nothing wrong with feeding a particular food if your dog does well on it and you feel comfortable feeding it, the question is whether you have a basis of comparison and whether the formulation of the food has changed over time. I have seen the effect a better food can have on my own dog. When I adopted him from the shelter, he was a thin little puppy with a brittle coat and a rather strong "doggie odor". I didn't know better yet, fed an average quality food and thought the change in his appearance was stunning, except for the severe reactions he still showed whenever he picked up the occasional flea and got bitten before it died. He had gained weight, the odor improved and his coat was softer and shinier. I was happy and didn't think that any further improvement was possible - until he had been eating a really high quality food for about a month. His allergy to flea bites disappeared entirely, the muscle tone became much more defined, his coat even glossier, softer and most important, much, much denser. The doggie odor vanished.
If I hadn't at least given the better food a try, feeding it long enough to see results (depending on the individual dog this takes about 4-8 weeks), I would still have been convinced that my dog "did just fine" on the lesser quality food. Now I clearly see the difference between "doing just fine" and truly thriving. Every step up the "quality ladder" will bring improvements, the stray dog who used to survive mostly on garbage will do better once he gets a daily ration of even a relatively cheap food because it supplies more essential nutrients; and a dog who was fed a grocery store brand is guaranteed to improve on a better quality product as well.