"We met a towering monster, wielding a sword of utter darkness. The Light only made it hungry. We tried to fight, and we fell. Too many times to remember."

Makes me think Crota may have some mechanic that 'eats' light or grants him more power based on damage from weapons of light.

We supposedly destroyed his sword. Well... we killed those who made it and it vanished. Is that the same thing as destroyed? For what it's worth, Crota is holding a sword in the preview videos. Maybe he had a backup or it less vanished and more was recalled to him?

Maybe the point of the Raid is to prevent a new sword is created/powered up.

That would certainly give the new Hive-themed, Dark-looking weapons a more interesting purpose.