No livro, eles fizeram.
Capítulo 4, Over Hill e Under Hill, página 64:
Suddenly a sword flashed in its own light.
(Gandalf empunhando Glamdring the Foe-Hammer / Biter)
página 65:
He took out his sword again, and again it flashed in the dark by itself. It burned with a rage that made it gleam if goblins were about; now it was bright as blue flame for delight in the killing of the great lord of the cave.
Por que eles não aparecem no filme, nunca são ditos. Não consigo pensar em nenhuma resposta no universo.