Allow me to interject, but I think the main purpose of Chaos for demons isn't spells, but rather skills that are enhanced by Chaos. The ideal approach is to first take Lord of Cold (as the chance of getting Ice Bolt is high), then Lord of Fire (to enhance Fireball or Meteor Shower), and finally, Drain Life (a very useful skill for a demon). And if you have an attack skill, it opens the way to Cold Steel (one downside is that it closes the path to Urgash's Call).
In this case, the skills Lord of Fire and Lord of Cold play a greater role, as they somewhat reduce the combat effectiveness of the enemy army. But in general, Chaos isn't so important if you're aiming for a super skill. But if you don't need it, the Chaos school can be very helpful.