Naruto never had to train to recieve the powers from Kyubi. The incidents with Jiraiya weren't training either, Naruto was just too stupid to figure out about Kyubi chakra until somebody told him. Other than that, it was the same concept of Naruto getting power out of nowhere whenever he needs it. Itachi actually had to train and master his sharingan, and even then, it's not as cheap as having almost unlimited chakra.