The Amurite world spell is useful if you are playing the Amurites and wat to build the towers. Simply build two towers, cast spell, and rebuild different mana nodes for the other towers.
The AI doesn't seem to know how to use it for any real purpose. The additional exp for the adept units even goes to waste by the AI. I have never seen the AI upgrade an Adept, and I have never seen them cast spells (other than world spells).
edit: The AI doesn't realize it, but when I see the Amurite world spell go off, they have just received a diplo modifier: "-12 You destroyed all my mama nodes!"