Rethban Ore was named after Bart Rethban, who discovered the ore and presumably the cavern it resided in. It's not as strong as iron, but it has a dark, flat color that some scribes use in their dyes.Brother Paxton, for example, has his own Rethban Ore ink recipe that he uses for inking copies of books.
Unlike many other quest items, it is lootable by players who don't have the related quest.
Although Rethban Ore is no longer supposed to be obtainable in retail WoW, players have reported finding it in random mithril veins, even in zones on the other side of the world. Since the quest is unavailable in retail, it has no use.