Are you sitting comfortably children...?
Well to start with I agree with all of you on Bombadil being a waste of space and I thought that when I first read LotR when I was 10 and I never changed my mind and generally skip the Old Forest leg whenever I re-read the Trilogy...
... but that doesn't mean he's not important in the scheme of things on Arda. Tolkien is a bloody nuisance over background and was always changing his mind and exceedingly vague about such unimportant but fascinating stuff like - can Balrogs fly?
(no they can't as too many of them appear to die falling off tall mountains) - what are little Orcs made of?
-(wouldn't we like to know? - ick! It's 'spirits' elves and later on men) - was Celeborn (Galadriel's hubby) a High Elf or a Dark Elf
(who cares he's still a drip) and I won't mention the Ents...
However he did say (in letters) what Bombadil wasn't
So - straight from the author's mouth - he's NOT
a) a Maia (like Sauron, Gandalf and Saruman)
b) a Valar (like Morgoth (Melkor) or Mandos (Namo) or Varda (Elbereth Gilthoniel) and
c) he's especially NOT
Eru Illuvator - the creator of Arda and so God
All Tolkien said was that Bombadil was the very first being on Arda before any of the aforesaid which has to make him a spirit of some kind. The Elves as the eldest sentient race in Middle Earth called him Iarwain Ben-adar which means Eldest and Fatherless - make of that one as you wish!
He lived with a minor spirit Goldberry who was almost certainly a Maia as she's the 'River's daughter' and the River is Uinen, also a Maia who was a servant of the Vala of the waters Ulmo.
We know he could 'do magic' because he rescued Frodo's friends from the Barrow Wight on the Downs and also the Ring held no power over him - which makes him more powerful than Gandalf because he was terrified of the Ring and wouldn't touch it with a ten foot barge pole.
So that's it really - he's some kind of unaligned powerful spirit who seems inherently benevolent. I think possibly Jason's right with the Green Man or some kind of Gaia for Arda who's more like a force of nature perhaps - but he's definitely not the Creator.
His presence/purpose in LotR is actually quite relevant to the story though intensely irritating in that he has to rescue the 4 Hobbits from Old Man Willow and later from the Barrow Wight. There's also a 'lesser' reason which actually is quite important to the nature of the Ring and to the story where Frodo - high on psycho-active mushrooms - dreams of Gandalf being rescued from Isengard where Saruman has imprisoned him. Not many people get the connection there, but it is an hallucination in which the combination of mushies and Frodo's anxiety for Gandalf (who was supposed to be with them) allows the Ring to manipulate Frodo's dreaming and shows him a 'true' vision almost in real time. So what? It means that the Ring was at least partially sentient and beginning to take hold of Frodo's mind. But not a lot of people know that...
Anyway - it's a really silly part of the story and not at all necessary which is why most directors (like Ralph Bakshi with the 70's animation and the BBC Radio version aorund the same time) like Peter Jackson cut it as it doesn't really add to the action or the main story...
... and yeah - I should get out in the fresh air much more often than I do!