Being Different is Scary, But Cool!
As to the notion that a person could be missing sufficient powers to work well in a team... well, every combat class experiences that to some degree. Poorly Armored Tanks, Mages wearing the wrong gear, Healers with the Wrong Stats, Spirit Bonding in Hard dungeons, and Pre-End game classes who forgot to complete their job quests and have missing powers. The Room for Error is larger on a Blue Mage, but it doesn't have to be as large as it might appear. Main Story quests and Class quests can have mandatory enemies, which means every blue mage will have at least a core list, probably +5-10 Abilities.
It can be scary at first to realize you might have to go learn better abilities, or at least face the harder version of a previous monster to get version II, III, or IV of a power, but it could parallel the growth of crafted items like Bone Staff->Decorated Bone Staff, or the gradual upgrades of Zodiac and Lucis items (or even the Allagan/Shroud upgrades). Different can be good.