Being MP-based means that you don't have to worry too much when it comes to running low on MP

You'll have the option to:
1. Use Ether potions (VERY useful since you can never replenish TP via potion)
2. Use Piety materia and/or accessories
3. Benefit from Mage's Ballad

Who knows, maybe they'll have a cleric stance-like ability that converts STR into INT? Since they'll probably be sharing the same end-game tank gear are the other tanks