Why was Tom Bombadil left out of the LOTR film trilogy?
To be fair, it isn't just Peter Jackson's movie trilogy that left him out. Ralph Bakshi did, hell even the 12-part BBC radio drama did! In fact, I don't know of a single dramatization that included Bombadil (with the exception of the unabridged audio book, if you consider that a dramatization).Why? Well, let's look at what the Bombadil section adds to the story. It introduces the concept of "ancient forest" and the idea of what will turn out to be Huorns. Bombadil is the first introduction of how regions in Middle Earth are run, and to the idea that there are relatively god-like creatures in Middle Earth who can interact with normal people. It's a demonstration of how relatively helpless the hobbits are at the beginning of the story (to contrast with the end of it), and like Tomas Snerdley mentioned, it allowed Merry to get his hands on a sword that will figure greatly in the end of the book.So here's the thing. You can figure out what the regions are like just fine by looking at Rivendell and Lorien, the hobbits are shown to be powerless when faced with the black riders, Gandalf himself makes a good intro to powerful supernatural beings, Huorns don't really need an introduction after meeting an Ent, and Merry could have gotten the sword anywhere between Hobbiton and Rivendell since that entire area was invaded by Angmar (although in Jackson's film, I think Merry used the knife he got in Lorien instead of the blade Aragorn gave him at Weathertop).And cutting out Bombadil cuts out about 20 minutes of run-time in a film that already has a theatrical runtime of nearly three hours without it.So as much as I love Bombadil, I gotta side with Peter Jackson on this one.