Regarding Arwen and Bombadil
Carl K.

Regarding Arwen, I agree that Arwen should have no role in the War of the Rings. The dramatic function of Eowyn's obsession with Aragorn is crucial to the outcome of the Siege of Gondor, because it is her dispair that drives her to masquerade as a young Rider of Rohan and ultimately slay the chief of the Nazgul, turning the tide of the war.

Regarding Bombadil, Tolkien says in his prologue that the tale grew in the telling. What initially began as More About Hobbits became an epic, and while he was finding his way into the story as the Hobbits were finding their way to Rivendell, he indulged in some diversions that have little to do with the greater scope of the Trilogy. Possibly at that point he had the idea to merge the work begun earlier, The Silmarillion, into the common-speech tale of the War of the Ring. In that more scholarly creation we meet the Valar; it is my feeling that Bombadil is Tulkas. Leaving him and other stuff out of the movie is necessary and in this case the opposite of critical.