Sojs were currency also because they were useful for any character that needs a boost of skill and mana, and due to packet sniffing in gamble, varies dupes (back in Classic, even).

Don't forget, no one would ever trade Elite quality items for Sojs. People had to use Ber/Jah/Cham/Zod to trade things like Cruel ____ of Quickness and Jeweler's ____ (light class armor) of Whale.