Needless to say, Glorfindel is an intriguing character, with a lot of depth. He is also, like Elrond, someone from an earlier age, and so filled with wisdom and experience of the earlier campaigns against evil forces in Middle-earth.

However, in reading LotR, you don't notice much of this. He is given only a few more lines than Arwen. Only in reading the Silmarillion, and then reading Tolkien's comments on the matter does the attentive reader realise that this is actually the same person who has returned. At the time of writing the appendixes, Tolkien attached so little importance to the fact, that he didn't even bother to write a short paragraph or two explaining the significance of Glorfindel.

So maybe leaving out Glorfindel isn't as tragic as it seems, seeing, it would require a lot of film-time to actually begin to get across who he really is. Why not have an anonymous insignificant servant of Elrond ride out to meet the travellers? That doesn't work, because it would have to somebody of strength and significance in case the Nazgul attack. That leaves Elrond, Legolas, Boromir or Arwen (assuming Gandalf was still away at the time). Elrond is a wise old king, and maybe too old for direct action. Boromir is maybe too prone to temptation, and lacks the subtelty of the Elves, which leaves only Arwen and Legolas. I would prefer Legolas, because there is no evidence in the books of Arwen actually bearing arms, but then if you want more women playing important parts, you have to go for Arwen.

PLEASE: I am not agreeing with Jackson's choice. I disagree with this as much as you do. I am merely trying to reconstruct his line of thought ....