I think this site explains why it's important for the toy breeds to stay longer with their mother.
Why do Maltese puppies need to be 12 weeks old before going to a new home?? By the way some breeders are keeping the longer because you can't have a valid bile acid test until 16 weeks. A good mama dog helps train the puppies for housebreaking. She licks them clean, so that they get use to being clean and she nudges them to the pee pads when she smells they have to urinate. She also teaches inhibition of the biting reflex, which is so important. As long as you are buying from a good breeder who keeps the mama with her babies, they will learn things you can't teach them so easily.