The Tolkien purists among us seem to have a major case of hostility at Peter Jackson for any tampering with canonical Middle-Earth, even though, as we know – and if we don’t, Qadgop will assuredly tell us! – JRRT was revising and reinventing Arda from the moment he invented it.
I humbly submit that he did an exceptional job of retelling the Tolkien story in movie format. And that, as I think we all know, is not merely, “Take what happens in the book and write a screenplay out of it.” Things need to be condensed, expanded, matters narrated in the book put into the mouth of a character so we have some clue what’s happening, side material omitted (good riddance to the Midgewater Marshes!)…
What Churchill might have called “the Battle of Rohan,” which the canon spreads over perhaps a dozen individual battles, is condensed into one. The bravery, derring-do-ness, and wisdom of various characters is established through having them do things in that battle that are not canonical (surfing-archery-elf-style, anyone?).
By the time we get to Helm’s Deep in the books, we know that the elves have been fighting an ages-long retreat against the forces of evil. We know they’re valiant, good, and doomed – and, like Vikings, determined to go down on the side of good.
But the movie elves are Spock characters, wise and aloof, Legolas to one side. We don’t have the background and narration that establish them as leading lights in the battles against Morgoth and Sauron for seven millennia so far. PJ needed to do something to say, “Elves are not just ineffectual nice guys thrown in for exotic effect; they’re an integral part of the War.” And he had precisely one place where their all-out commitment to good vs. evil can be displayed – and the misconception of their aloofness dispelled. So he sent Haldir and Lorien elves to Helm’s Deep.
Contrary to the LOTR book text? Yes. Contrary to Tolkien’s conception of who the elves were and what they stood for? Not in the slightest.