The wrong kind of details: the components of wonder (aka lmp's rant)

[The Squatter of Amon Rûdh]

It’s this creative authority, I think, that allows the prof to get away even with such outlandish details as Tom Bombadil, who, in any other work, I think, would smash credibility to tiny bits.

On the other hand, does he need to get away with Tom Bombadil? I rather think that part of the wonder of Middle-Earth is that it's impossible to know or explain everything about it. We're intrigued by a character like Tom because he can't be quantified. In a world where more and more things, including people, are being explained, defined and conveniently pigeonholed every day, we're rapidly running out of opportunities to experience the thrill of the inexplicable in adult life, and therein lies the attraction.
Not only that, but I would find it pretty difficult to believe in a world in which everything was conveniently explained for me: some things are best hinted at, as with Tom's brooch from the Barrow Downs, for example.

Having said that, some people (and I must admit that I'm one) like lots of additional information that would completely ruin the narrative, so, true to his academic training, the Professor put all of that background into a series of appendices, to which the curious might refer without the need to break up his story with footnotes and long-winded digressions. In so doing he gave his world depth without drowning his narrative in details, and still left enough unsaid to keep the reader interested. Small wonder that Tolkien has so many imitators and so few equals.