No one food is good for all dogs, but with this range it should cover most dogs. Rather than the protein level, the source of the protein is what is important. Chicken, turkey and lamb are the lowest in purines. If a Dalmatian has urinary problems then it usually means that he can't break down purines in his diet.
Scraps help make the dog's diet more complete.
When we used to slop the hogs and slop the dogs, we didn't have hip dysplasia or many of the other more modern structural problems. Our convenient dog foods are an improvement in many ways, but they are not complete or balanced. Veterinarians began recommending not feeding human food when the average pet was found to be too fat., Moving from the farm to the city and from work to retirement caused many pets to loose their shape. Feeding too many scraps isn't good if it adds weight to dog, but there are just too many nutrients that go down the drain or in the trash that are helpful and useful to our pets. The foods are also shipped long distances and sit on the shelf sometimes for months, are exposed to heat, etc.