I think the point is that the Istari were sent to provide advise to men not to
directly challenge Sauron. If the Istari had there full powers they
may well have wrecked ME. ant therefor they where sent with the bodys of men.
Sauron was able to wrap himself in human form and if that body was destroyed he
could remake it, because the essential Maia sprit continued to live.
Gandalf on the other hand was given a real human body which could and did die.
I think that this human body could not be reformed as such and he would have
had to be given a new although very similar body. (Though as a side note if
Gandalf were to come to ME as his real Maia self then I think he would have
been able to adapt any shape he liked, very much as Sauron
did)
