Actually in the FOTR Book 2 Chapter 1 it says: "Soi t was thatFrodo saw her whom few mortals had yet seen; Arwen, Daughter of Elrond, in whom it was said that the likeness of Luthien had come on earth again; and she was called Undomiel, for she was the Evenstar of her people."
IMAO I think that Tolkien wanted her to resemble Luthien as much as possible, and having Aragorn tell Frodo about Luthien in the book, instead of actually telling him about Arwen, was his way of telling the readers about Arwen w/o actually leaving Frodo's story. IMO!