I don't think it defense, for the white mage. It looks like magic enhancing in general. Whether it works with other casters is something we'll find out another time.
Looking at the video, Aero, Stone and Holy all three look different. It doesn't make sense to me that White Mage would get 3 new damage spells together with that field. It makes more sense to me if that field enhances all spells (effect pending) being cast in the field - Which also makes sense considering they're conjurers and the lore.