There's also one reason that makes me think the kyuubi is important.
I believe the choice of Jiraiya as Naruto's teacher wasn't a random one. Jiraiya is the frog hermit, and teachers Naruto how to summon frogs. If we look at frogs, we can say that, they are maybe the only animals who transform so radically during their life time, they even have to change environment to accomodate their growth.
In the begining, Naruto was only able to summon tadpoles, it took him a life or death experience to learn how to tame the kyuubi power and summon an adult frog.
So, IMO, the frog is quite an appropriate animal to symbolize Naruto's transformation. In the moment he becomes aware of his immense power and find ways to tame is not a child anymore. He is able to deal with his power like an adult would.
So we have two animals to make Naruto's transfomation pretty clear, the frog, who, as a tadpole represents Naruto as a child and as Gama-bunta, representing Naruto an adult , and the Kyuubi, the demon-fos symbolizing the power that Naruto had to tame to become an adult.