Why Moria?

[Inziladun]

Originally Posted by Rhod the Red

Umm....say maybe Radagast could come to personally bring news of the High Pass being unusable, the Company ask him to come through Moria with them.....do you mean like reasoning like that or something?

Why would Radagast have been more knowledgeable about the High Pass than the people of Rivendell?
Gandalf's presence in the Fellowship s perfectly plausible, since the Ring had been his focus for a long time, he had been involved with its finding, and he was very intimate in counsel with the Elves. Radagast suddenly getting into the Company would have strained credibility. And why would we need more Wizards anyway? The idea was that of the Istari, only Gandalf had truly remained faithful to the original mission, which was the reason he figured so prominently in the story as written. Radagast did his bit in getting Gandalf to Isengard to bear witness to Saruman's treason. He then apparently went back to the woods to play with the rabbits.

[Zigûr]

Originally Posted by Inziladun

The idea was that of the Istari, only Gandalf had truly remained faithful to the original mission, which was the reason he figured so prominently in the story as written. Radagast did his bit in getting Gandalf to Isengard to bear witness to Saruman's treason. He then apparently went back to the woods to play with the rabbits.

I think this is a really crucial point. Radagast's role in the narrative of the Wizards is important in my opinion because it shows that the Wizards' failings didn't just involve becoming evil like Saruman - it could also mean becoming reclusive and apathetic. I think it's important that after sending Gwaihir to Orthanc Radagast disappears. Having another overtly heroic wizard like Gandalf would, I think lessen his impact and reduce some of the book's potential suggestions about the desperate situation of "good" in the world.
I think the rather curious remark that Radagast could not be located later on is one of those devastatingly subtle points Professor Tolkien makes so effectively in his writing.

[Nerwen]

Rhod, I just can't see what this would accomplish in story terms– except, of course, to add a quite unnecessary extra plot complication.

[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.

[Rhod the Red]

Well maybe he wouldn't have been useful in Moria. The Balrog might have killed both R* & G* as opposed to just one.

[Mithalwen]

Radagast in his limited appearances in the books doesn't really seem either particularly qualified for or indeed up for much in the way of adventure. He is not a traveller save at great need and scarpers just about the very second his message is delivered. His journey to the borders of the Shire was surely via Rohan so he quite possibly had little or no knowledge of the mountain passes. Certainly unlikely to know more than the elves who live in proximity or those like Legolas and Gimli who presumably had come that way.

And he had inconveniently disappeared.

The western side of the Misty Mountains was more discreet and given the sense that is given that the ring has some control over its fate maybe it was thought unwise to take it back near to the places where it had effected two transitions of owner. Maybe too fanciful. But certainly the Western route tooke them relatively directly between probably the two safest places in Middle Earth protected as they were by Vilya and Nenya as well as the inherent strength of Elrond and Galadriel. The route had been scouted by the sons of Elrond and the first part quite likely had a measure of protection from the Rangers who still had settlements in the Angle.