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Okay, each demon gets a consumption and control rating, instead of just saying that this one damages the body quickly or that one is really hard to control. Both the amount of control that a user has and the rate at which it consumes it's user are important factors in how long their powers can be used. For example: Gaara can activate and take control of his sand at any point, and it's safe to assume that this is a release of his demon's power. He can use that for a very extended period of time because he has more control over it and it does not consume him as fast. On the other hand, Naruto retains his own mind when he releases the single tailed Chakra Cloak, his first release stage, but not much of his own actions and the cloak actively consumes his body. By the second stage he looses control completely and his body is actively eaten faster than the rate that he can heal, while Gaara's second stage release has him still retaining his mind and allowing him to act mostly of his own free will.

They are important parts of the demons and would allow the demons to be more balanced. Kyuubi might be the most powerful, but it would naturally be offset by how fast it consumes it's host and how little control the host has. With this in place there wouldn't be any reason to say that Kyuubi is too powerful and no one can have him.