A reader sent me the link to an article on a Royal Canin subsite today:
http://www.petfoodnutrition.com/no-guts-no-glory-getting-real-about-by-products/

Actually I have written about this very topic in great detail here:
http://www.dogfoodproject.com/index.php?page=ingrd

In a nutshell – I am a raw feeder and feed my own dog lots of items that are considered byproducts, right down to chicken feet. However, fresh byproducts from animal carcasses inspected and passed for human consumption, which are properly refrigerated or frozen throughout the production chain are a far cry from the kind of byproducts the pet food industry renders into meals, and even non-rendered ones can vary in quality very much.

The only way I ever would trust any inclusion of byproducts in a food is if they were used fresh, individually declared (e.g. lamb heart, beef tripe, chicken liver) in the ingredient list, and the company made a statement about the level of quality.