I don't know why, but has it ever occured to people here that Kyuubi isn't as invincible as people make it out to be? What I mean to say is, maybe Kyuubi is a double edged, sword. And by tapping into his power, Naruto ultimately loses a part of himself in the process. I dislike the idea of having an invincible trump card for Naruto, mainly cause he's such a clever fighter, and will use trickery to win, instead of brute force.