Expanding on what Tenhou said, Mage Armors have inferior physical defense to Heavy Armors but superior MR and Jessica's Fireball skill is great for clearing weak mobs that can end up sneaking past even HAs (due to their low attack this can happen with large groups if the ranged support is either not there or single-target). Higher UP cost is a thing to keep in mind, though.


For reference:
Jessica CC70: 1980 HP, 456 Defense, 15 MR, UP 30
Jessica AW90: 2891 HP, 570 Defense, 15 MR, UP 30

Leanne: CC50: 1931 HP, 507 Defense (811 for 30 seconds), 0 MR, UP 18-21

Even a silver HA is comparable in survivability if enemy is purely physical (huge HP disadvantage against AW Jess, but her defense can go up to about 1.5x Jess'), but all Heavies lose badly in offense (except Deine if enemy numbers are low) and MR (Gellius only loses by 5 after AW)