[JennyHallu]

Perhaps the staff, like the One Ring, is a repository for some of the power of the Istari, as well as a symbol of office. After all, both Sauron and the Istari were supposed to have been Maiar, and perhaps physical repositories for metaphysical power were not uncommon among those people.

The breaking of Gandalf's staff fighting the Balrog may have been a sign that he had had to use all of the power he had stored there (for easy retrieval, perhaps) was used in the conflict, and he had to fight with power from his own person. The replacement of his staff upon his return to Middle-Earth could have simply been intended as an outward symbol of his new status as the Head of the Order. Notice that where up to that point most of Gandalf's displays of power were done by use of the staff (the fire on Caradhras for example, and the light in Moria) he was much less dependent upon it after his fall and rededication, but he still used it occasionally, and not when he was fighting very sophisticated deviltry (i.e. In the hall of Theoden King, casting down Grima's mental, not magical, influence)

And after Gandalf breaks Saruman's staff, Saruman's power is not negated. Weakened, yes, which would follow being rather dependent on a staff that had had millenia to absorb power, but even without his staff he was still capable of wreaking terrible havoc in the Shire...

Just a thought...