Had PJ not done so, the climax of TTT would have been utterly stupid, and the whole movie would’ve fallen apart: some strange bloke named Erkenbrand would have appeared with a huge army to save the day, then he’d never up again. I can see the reason for altering that, just as I can see why replacing Glorfindel Otherwise Not Appearing In This Film with someone we know (who needs to do more than sit on her butt) was a good theatrical move.
PJ’s move to give the Elves something to do, and meet the “Why don’t they help?” complaint that readers of the books often make, is far more controversial. But without them, who would’ve held Helm’s Deep until Éomer arrived? And without Éomer being “away” for most of the film, who would’ve fulfilled the Erkenbrand role?
There is a second reason for keeping Éomer out of the limelight. Every time you add more heroes to the pot, it lessens screentime for all the rest, and makes it far more complicated trying to give everyone something to do. Merry, Pippin, and Gimli constantly suffer from this problem. Even Théoden spends a lot of TTT standing up on the observation deck making dramatic pronouncements like, “Is this all you can do, Saruman?” while in fact he is doing precisely nothing, and Aragorn & co are getting creamed. PJ wanted to show Aragorn growing into the role of leader so his audience would believe it in the third film. Had Éomer been around with all his men, he would’ve been up there organizing the defense, rallying the troops, shouting orders, and Aragorn left in a rather anomalous and useless position with no authority to do anything much.
I wish PJ had found some way to stay closer to the book while addressing the issues I raised above, but I can definitely understand why he had to make some changes to deal with these problems.