i disagree rebel, at helms deep in the books the duenadain or what ever the guys are called,(rangers) come to aid rohan, and with them came the sons of elrond as well, in the movie it would have been very confusing if they had brought in the rangers and introduced the new characters, so instead they brought in the elves inorder to give the elfs more importance they used the elf from lorien as the leader because ppl knew him from the last movie. so all they change was the identity of the ppl who came to rohans aid and if you say that is doesnt fit, then take it up with JRR tokein cus it was his idea.

i personally would have been appalled if they had elfs come to the battle in ROTK. it would have gone against the book completely. also remeber that the ROTK battle was the big battle for middle earth, the elfves said that they didn't want to get involved, because it was nolonger up to them to save the day, the race of men has to win this battle by itself.