Radagast gets only a very little mention in the Hobbit-
Gandalf mentions him to
Beorn and says that his dwelling is nearby, and Beorn replies that R is not such a bad fellow, as far as
wizards go. This was all written long before
Tolkien had any idea what the wizards were- while inventing 'The
Hobbit' for his children Tolkien would include bits from his greater history of the
Elves, but he had not at the time (~1920-1936) firmly defined the relation between 1st and 3rd Age
Middle-Earth. While writing the first chapters of
The Lord of the Rings (up to
Moria) Tolkien was sticking more closely to the formula of the Hobbit, which is why some of the earlier episodes might seem reminiscent of the earlier book-
Mirkwood and
the Old Forest, the
Wargs and the other Wargs,
Rivendell, etc. In this setting Radagast makes his onely appearance in LotR, and I must say he looks a little weak for one of the
Ainur. In my opinion JRRT seems to forget Radagast later on in the tale- in fact he has
Treebeard make the improbable statement that Gandalf is the only Wizard who ever cared for
trees and
plants! To me (and apparently to CT as well) Radagast wasn't so much a failure- while the other
Istari were sent more explicitly to help Elves and
Men, Radagast was sent by the urging of
Yavanna to help the olvar and the kelvar, a job which I think he accomplished quite well. Why was he a failure? CT asks much the same question in Unfinished
Tales, and I don't think there is a satisfactory answer. Was he necessary? Not exactly- but I think the mis-balance in the story created by his actions is far less than the ones introduced with (say) a
Glorfindel or the
Eagles.