Some people are very passionate about their pets. When it comes to training and food, some people can come across looking like zealots, with information looking like it's based more on beliefs than on facts. What makes this documentary interesting is that it helps teach the viewer some of the history of the pet food industry, how certain products have evolved and come into being, and how to read labelling on pet food packaging so that you can pull out the information you need to make an informed decision, and (perhaps more importantly) to identify the BS terms pet food manufacturers use to present their products as being more than what they actually are.
For every vet that presents one idea, there will be another vet that will claim that said activities are dangerous because of x, y, z. Unfortunately that means that the onus is on pet owners to find vet that they trust the opinions of to help them to make informed decisions on what to feed their pet(s). This documentary (along with The Gruen Transfer's episode on Pet Food) should be essential viewing for all pet owners, especially given that the Pet Food Industry is not regulated at all and thus they can say what they like on their labels.