[The Might]

Well, probably since Glorfindel doesn't do much else in the book, PJ thought he might present Arwen as the warrior princess. Remember she not only steal Glorfindel's part, but also Elrond and Gandalf's by making the Bruinen grow and hit the Nazgul.
Arwen's part definitely became more significant this way, especially also keep in mind in the movies she is the one without M-e would have been taken by Sauron, since without the sword (which in the movie is only made at Arwen's request) Aragorn couldn't summon the Dead, and all that stuff.