Well, you don't HAVE to. Since I played as a magician and allied with orcs in my only play through and I didn't have any problem or limitation that would bother me. But yeah, to learn some ( not very useful ) spells, you have to be high ranking ally for rebels I guess. Since there was no one among orcs to teach them.
But you can get mana regeneration regardless which side you take and that's all that matters for a mage in Gothic 3 IMO.