not really a complete theory yet.... I believe the keyblade is actually more powerful than sora realizes. without the keychain it has much more power and can even transform as seen in( YouTube - Terra animacion kh3 ). The keychain allows a wielder to use certain aspects of the power. I think even mickey might be afraid of this power and therefor he uses a keychain as well. TAV, MX, and DS are skilled enough to unleash and control this power without a keychain.