Here's my take on this....I have had dogs my whole life and they did just fine on the cheapo brands, meaning they lived long (healthy) lives but as with most things in life, priorities change. I don't like paying high Vet bills and if I can feed my pets(dogs) a better, more nutritional diet(without going the ah-natural route), I want to do that in hopes of sustaining their life as long as possible and keeping them as healthy as possible without frequent visits to Dr. Doug. (no offense DR. Doug...lol)
I'm not certain, but its floated that feeding a "better" grade food may provide for a longer/healthier life. I suppose I buy into this notion about healthier pets with better food! (whether its true or not??). I fully realize its a very COMPLEX" issue but just as some of us rely of financial advisers to invest money, I look to some others who have more knowledge about such things as "by-products", "Organ Meats" or whether those pet food manufacturers have any truth in advertising.
I'll end with this, some 50+ years ago my brother went to a meat packing house that produced Hot Dogs...after observing the process of by-products from organs, hoof's(what have you) being scraped off the floor and thrown into the mix....he didn't eat hot dogs for years after this. In the same vain, I'm quite confident that if I visited a Dog food plant, I'd be fairly disgusted with the practices taking place their too!
Long story short, I guess I have my answer, there is no definite source, its all open to interpretation or mis-interpretation of what is good or bad in the world of Food source in general. Oh well...:-)