Let's start with the most jarring aspect of the movie Helm's Deep scene: the elves! So said Elrond in the Council:
> Never again shall there be any such league of Elves and Men; for Men multiply and the Firstborn decrease, and the two kindreds are estranged. And ever since that day the race of Numenor has decayed, and the span of their years has lessened."
Haldir:
> we dwell now in the heart of the forest, and do not willingly have dealings with any other folk.
It is indeed perhaps the biggest fault of the otherwise excellent movies. Replacing Glorfindel with Arwen didn't bother me at all, for example.
True, and this is emblematic of the changes in the Two Towers that easily made it the worst of the series for me. Though, I can empathize with why some of these changes were made.
The text of the Two Towers as it exists on the page has practically no elves in it. We are shown wanton destruction of the Westfold and the immense suffering in Rohan, yet we must buy they will soon ride to the aid of Gondor. The Battle of Helm's Deep is pitched as an existential fight for Rohan, and to have the Elves sit this one out would likely come across as extremely self-centered to modern audiences. Whatever the Elder Folk may have suffered in the past, they are the most organized and best equipped in Middle Earth. To have them sit out the destruction of Rohan in their literal Ivory towers would probably alienate audiences.
> Never again shall there be any such league of Elves and Men; for Men multiply and the Firstborn decrease, and the two kindreds are estranged. And ever since that day the race of Numenor has decayed, and the span of their years has lessened."
Haldir:
> we dwell now in the heart of the forest, and do not willingly have dealings with any other folk.
It is indeed perhaps the biggest fault of the otherwise excellent movies. Replacing Glorfindel with Arwen didn't bother me at all, for example.