Eru Iluvatar have mercy upon us!!
Look, the entire point of making a movie out of a book is to adapt the story as told in the book to a story-that-works-in-a-movie. That means that if there’s an internal monologue, the protagonist thinking something, in the book, one must either do that as him sitting/standing with a voice-over of him thinking, or rewrite it so he has those thoughts in dialogue with another character. It means if there are references to repeated events “offstage” in the book, one must either bring them onstage, drop them, or consolidate them.
The Battle of Helm’s Deep was used by Peter Jackson to consolidate the Battles of the Fords of Isen, half-a-dozen battles across half of Rohan, and numerous other events, including the Lorien Elves attacking Dol Guldur.
According to the book, the Elves have been a vital part of the warfare against Sauron and his forces since the Second Era, over 5000 years before. But all that happens “offstage” in the plotline of the books. So if we’re going to have any Elves besides Legolas shown as fighting the Sauron/Saruman alliance, they have to be doing it onstage in the movie, and, since PJ decided not to have repetitive battles in The Two Towers (in which I think he was right), that means you’re going to have to show them taking part in the one battle shown – Helm’s Deep. As an adaptation of the story to meet movie conventions, it was a bright move on his part. We see the Elves not just as mystical, place-out-of-time figures, but as active parts of the coalition of the good guys, actively combatting Sauron. And we know from the books that Lorien and Rohan were in fact allied, though there was some superstitious fear of elves among the Rohirrim-in-the-street and some sense of men-are-taking-over-and-we’re-left-behind among the typical Elf-in-the-woods.
Likewise, the Tale of Aragorn and Arwen is canonical, but placed in an appendix as not being integral to the Quest Story that forms the major plotline of LOTR. In order that there be some female presence in the story, PJ moved it into the main story line, and reworked Aragorn’s motivation a bit – a plotline that makes a bit more sense out of why he’s waited until now to claim his inheritance. This was truly a departure from Tolkien in the way that the consolidation of battles into the Helm’s Deep scene was not, but IMHO was a change for the better. Don’t forget that Tolkien consistently fiddled with the story lines of his mythology over a sixty-year period to attempt to make it more in tune with human nature (and his conceptions of Elf, Dwarf, etc. nature).
What I’m saying is, in a movie adaptation, it makes more sense to consolidate the battle scenes, which take far longer to show than they do to be alluded to in the book, into one climactic battle – and if you do that, you need to incorporate all the developments of the various battles into that single battle scene. Ergo, Elves combatting the Forces of Evil militarily, which is canonical, must be brought into the one battle you’re going to film – which means they come to the aid of the Rohirrim (as, if you read the history, they actually did in other times and places).
PJ could have filmed the story exactly as Tolkien wrote it – and produced a 20-movie-long series that would never have been financed and would attract an audience of exclusively rabid Tolkien fans (who would then have found other things to bitch about). And he’d have never gotten it off the ground.
Personally I think he did an outstanding job of remaining more-or-less true to the canonical plot while adapting it for the big screen.
