I don't know how true it is, but I read it here:
Starting Your Puppy On A Raw Diet | Dogs Naturally Magazine It’s not wise to mix your puppy’s raw food with kibble. Because kibble requires a different pH in the gut to digest, it will make your puppy more susceptible to the bacteria in the raw meats. He is capable of handling this bacteria just fine, but once you add in artificial foods, the meat will sit in his digestive tract twice as long, meaning there is a much greater chance of harmful bacteria building up.
Maybe it only pertains to puppies...?