I've thought about Bombadil a bit. I've also posted this in a different thread, because I figured more people would read it that way.
I have a little theory about Bombadil, and it goes like this.
e may be an anomaly, but in my view he is quintessential to the books, because he shows Tolkiens world is essentially one of powers.
This is evident when Gandalf tells Frodo about his imprisonment in Orthanc.
('Yes, I, Gandalf the Grey,' said the wizard solemnly. 'There are many
powers in the world, for good or for evil. Some are greater than I am. Against
some I have not yet been measured. But my time is coming)
This is defined in a spheric, or topical world way. Each 'power' (this is of course the Valar and Maiar, but also the Istari and the ringwielders) has its field of influence, or care/stewardship. This is evident in Rivendell and Lorien being places where the decay of the world is halted for some time, and the memory of the ancient days of Middle-Earth can still be felt. Elrond and Galadriel both have, through their rings, a stewardship of sorts, but of a physical place.
Gandalf is also a steward, but in a more general sense: he is a carer for the children of Iluvatar. He says exactly this to Theoden on his suicide rampage ('I too, am a steward')
From the Silmarillion:
Wisest of the Maiar was Olórin. He too dwelt in Lórien, but his ways took him often to the house of Nienna, and of her he learned pity and patience (...) though he loved the Elves, he walked among them unseen, or in form as one of them, and they did not know whence came the fair visions or the promptings of wisdom that he put into their hearts. In later days he was the friend of all the Children of Ilúvatar, and took pity on their sorrows; and those who listened to him awoke from despair and put away the imaginations of darkness. (Silmarillion)
He is after all, ‘a servant of the secret fire’ and ‘wielder of the flame of Anor’ (which is the ring Narya, the ring of fire):
It is described as having the power to inspire others to resist tyranny, domination, and despair (in other words, evoking hope from others around the wielder), as well as giving resistance to the weariness of time: "Take now this Ring," he said; "for thy labours and thy cares will be heavy, but in all it will support thee and defend thee from weariness. For this is the Ring of Fire, and herewith, maybe, thou shalt rekindle hearts to the valour of old in a world that grows chill" (Círdan the Shipwright to Gandalf)..
Being a power (a good power at least) essentially involves being a steward. Look also at Melian and her girdle. Aragorn who spends many years protecting the Shire and other parts of the world.
Saruman who becomes steward (and eventually a locked-in gatekeeper) of his own fortress. This is true for Sauron as well, who wields his power in the form of influence (eg. pulling snow from Caradhras). His servants perform the physical acts. All the more due to his no longer having a physical body. And take Denethor, he is no more than a steward. As opposed to Theoden who is a steward to his people ('what will you say of the children of Rohan, who lay dead at helm's deep' as he says to Saruman.. imagine Denethor saying that).
To me, Bombadil is so important because he symbolizes exactly that. By showing us the gentle but all-encompassing power Bombadil wields when he is within his own domain (evident because he is unaffected by the rings power, and has power to hold Old man willow and the Barrow Wights), Tolkien in a way shows us how and why these powers exist in the world. As has been remarked here, Bombadils interest and influence go no further than the borders of his land, the boundaries of his sphere. Therein he is all-knowing and omnipotent (again, in a gentle way).
As such he is to me Tolkiens way of illustrating the way powers manifest themselves and interact in his world. Bombadil also makes us see the difference in the way the good and evil powers manifest themselves. Was not the fall of Melkor a result of the fact he was not satisfied with where the limits of his influence or care lay, though they were the broadest of all the ainur. His very aim was to reshape the world as he saw fit (changing the songs of the other ainur
, destroying the lamps ). But when the Valar retire to valinor after the destruction of the lamps, he literally dilutes his essence, his power all over Arda, marring everything. Hence, he is not strong enough to withstand the Valar at the time of his first imprisonment (in the Halls of Mandos).
"The Morgoth" was a term given to the person of Melkor/Morgoth in his complete power over the matter of Arda: therefore Dragons, Trolls, Orcs, and even Angband were in a way part of "The Morgoth", but not part of Melkor/Morgoth. While Melkor/Morgoth was eventually executed by the Valar, the only way to destroy "The Morgoth" would be to completely destroy all of Arda and render it anew: a task the Valar could not do without also destroying the Children of Ilúvatar and therefore unthinkable.
source:
http://www.indopedia.org/Morgoth.html#Names_and_Titles
That's why the valar can only banish his fëa and cast it into the void, instead of destroying him. (see also:
http://tolkiengateway.net/wiki/F%C3%ABa). So in this way Melkor as a power has gone the furthest, dispersing himself so his influence is felt everywhere, but at a high price.
But I digress. I think the notion of stewardship is essential here. Being a steward not for one's own good or to counter one's own fears (Saruman, Denethor), but for the good of the world. This is of course a very Christian thing and biblical (man is a steward to world) which is not surprising given Tolkiens background.
To me Bombadil illustrates this so well because of the small sphere of his stewardship. And his comical reaction to the ring. He is the only incorruptible power (remember, Gandalf wouldn't touch the ring), save Illuvatar perhaps. Of course this supports the theory he is (in the logic of the tale) the embodiment of Illuvatar. But.. I think it's better to interpret him as an enigma illustrating some essential points of Tolkiens world and philosophy, then to try and force him into the laws of logic inherent to this world. Tolkien said something similar in one of his letters (can't be bothered to look it up, sorry
).
Too bad every adaptation of Tolkiens work (all movies, radio plays etc.) I know of has failed to see Bombadils significance.