Of all the classes, White Mages are most likely the most important jobs to have. Even if you have a load of powerful attackers, they need healing and the Red Mages lower level healing, as well as potions, are just not enough in the later dungeons.
Plus, after much research in FF1 and later titles, I discovered that while the White Mage has lower defense and attack, she has a pretty good speed stat, as well as a defense higher than that of a Black Mage. So she often dodges more attacks than the BM, and receives less of a beating when she does get hit. Why do you think White Mages stands in front of Black Mages during a combat situation?
So, while White Mages are no good when it comes to taking on enemies, she is more often the only thing making sure her allies can keep up their attacks. Every class has their place, and a White Mage's place is to keep herself and others alive. What's a powerful party filled with great strength and black magic worth if they're all dead?