[Nerwen]

I mean, Rhod: I do understand that from your point of view the "purpose" would be simply to get a Radagast cameo... but you see, some other reader might have liked Tom Bombadil to show up again, or Glorfindel (or hey, Arwen riding Asfaloth...) In fact, what you're talking about would be one of those writing placeholder things: "...and then they can be guided through Moria by [insert character name here]". That is, even if Tolkien had decided to throw in an extra character at this point, I really can't see why he would have, or should have, chosen Radagast in particular anyway.