A lot of dogs do well on the foods widely available in supermarkets, my first ever cocker would indeed only eat pedigree complete, so they cant be all "bad".
But saying that when you do look into whats in them, who makes them and their animal testing practices (in some cases) then you do tend to want to move away from feeding them (well I did).
The one thing aside from the animal testing is that all the big names are made by food processing companies and they make dog food to get rid of the by products from that industry and make a huge profit. Did you know for example that kibble is sprayed with old cooking fat so that dogs will think its tasty?
Hills is sold in vets as during their training they do animal nutrition - and guess which company it is that goes in and does the talks? Vets are just led (like us) to believe that its the best food for dogs
The smaller companies such as Arden grange, Burns and Naturediet don't have hidden agendas, they don't have a lot of waste by products to use at a profit, they don't animal test with vivisection.
It isn't suprising that dog food has only been sold since the supermarkets took off. Before humans started eating processed foods, dogs got whatever was going