Well, since you stated:
"The staff itself have limited magical powers, but is filled with the magic of the wizard.
But my theory is that it's used to channel Gandalf's own powers, it can't create magic on it's own. Like a sword that doesn't kill if there's no-one to wield it. But it can't contain an infinite "mass" of power."
and esspecially
"
I'm not saying that the staff is some ordinary piece of wood, collected in the nearest forest. Just like a king's sceptre or a great warrior's sword it's a very special item, although it's not the staff that makes the magic. It's probably magic in itself, enhanced to make it withstand immense power, fire etc. and to help the wizard focus his powers. But once again:
the magic a wizard creates is his own and is not absolutely dependent on his staff."
then I see no disagreement between us - quite the contrary. As Tolkien states, "magic is an inherent power" in the wielder, but the staff isn't your everyday piece of wood.
EDIT: How could I forget?

. Another even related to the staff's importance, even after Gandalf is the White, is the Theoden's hall incident:
Quote:
- The staff in the hand of a wizard may be more than a prop for age, said Háma
...
He raised his staff. There was a roll of thunder. The sunlight was blotted out from the eastern windows; the whole hall became suddenly dark as night. The fire faded to sullen embers. Only Gandalf could be seen, standing white and tall before the blackened hearth. In the gloom they heard the hiss of Wormtongue's voice:
- Did I not counsel you, lord, to forbid his staff? That fool, Háma, has betrayed us!
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Now, what makes that piece of ash-wood worth risking the entrance into the king's hall at such a critical moment? And why did Grim give it so much importance (and Hama too)? I would say, again, that the staff has certain magical capabilities of its own.