

My devil’s advocate argument for the elves being there is that there were a bunch of battles in the north that didn’t make it into the movie and only get mentioned a little in the books, and one of the important themes of LOTR is that all these disparate groups had to band together to fight Sauron. So having elves be at Helm’s Deep is a way to show the different people fighting together in a movie series that was already pressed for time. Necessary? Maybe not. But it doesn’t bother me as much as some of the other changes, because I can at least see a rationale for it.
Username checks out.
Anyway, I think the actual answer is compassion.