In combat the supportive abilities are more important because you shield your whole team and the offensive abilities can be tide turning if the enemy has a weakness to the element in said attack but usually that's only bosses like dragons. The basic attack for the mage is the worst out of all three doing under 75% the damage of a bow on the same level. Out of combat the mage is essential in solving puzzles, creating paths and even completing quests which is honestly quite stupid because the rogue should be the class that's better for exploration.