Yeah, but he could've had a change of heart when he realized that natural energy will help him recover/protect his body from the Demon Fox's Cloak. If he can use Kyuubi's chakra and still stay completely conscious while doing so, AND without causing any harm to his body.. why wouldn't he use it? The same can be said for Rasenshuriken as well.
Btw, I really think it is Rasenshuriken.. but recently I've been thinking about how it could be Kyuubi's power.
Cause untill 3 tails he is conscious and doesnt damage his body. After he is not conscious, but he is invincible.
IF he learns how to be conscious @ 4t mode and at the same moment not damage his body, any upcoming Naruto fight will be pointless.
He'll use that cero and kill any opponent... ah yes. Hes invincible too )