James Bow refers to what he calls the Glorfindel Syndrome, which appears to be what happens when the law of economy of characters is applied to a sprawling epic with a huge dramatis personæ when it’s adapted for the silver screen:
The name is taken from J. R. R. Tolkien’s [elven] character of the same name, who rides out from Rivendell to meet the Hobbits and Aragorn, and takes Frodo back to the Elvish kingdom, facing down the Dark Riders along the way. In Peter Jackson’s movie trilogy, Arwen takes Glorfindel’s role, giving her valuable extra screentime. In the 1970s animated version, Legolas steps up to the plate, for roughly the same reason. Glorfindel is, basically, chopped liver.
During the heyday of the Peter Jackson movies, I actually conceived of a related idea: a Web site protesting the exclusion of Glorfindel from movie adaptations of The Lord of the Rings, replete with foaming invective against the characters that replaced him. (For example, impugning Arwen’s racial purity — she’s only 78.12 per cent Elf!) I never finished it, probably because I couldn’t make it funny enough to make it worthwhile. I also imagined it as a satire of nitpicking fans (the kind who were upset at the omission of Tom Bombadil or “The Scouring of the Shire”); there was too much risk of it being taken literally. So it never came into being.