
The way I look at it is there is no longer any White Magic or Black Magic like there was in FFXI, it's all just elemental magic. Even Cure is considered Wind Elemental. So it doesn't seem like that big of a stretch if you have CNJ/WHM and THM/BLM to claim your character has mastered all the elemental forms of magic.
Likewise, a character who starts as a MRD/WAR and then trades his axe in for a spear to go LNC/DRG could also claim to be a master of both. I don't think we'll see a lot of people claiming just because they have every class at 50 they can RP as being a master of every class (but I could be wrong), but I don't think multiclassing should be considered taboo in general. Plenty of characters in game tend to bend or break the rules, such as Thancred supposedly being a Bard but he uses a weapon not even available in the game (throwing knives), or Y'shtola being a White Mage or Conjurer but having crazy psychic powers (or at least her spells in cutscenes appear like psychic powers).
Likewise, a character who starts as a MRD/WAR and then trades his axe in for a spear to go LNC/DRG could also claim to be a master of both. I don't think we'll see a lot of people claiming just because they have every class at 50 they can RP as being a master of every class (but I could be wrong), but I don't think multiclassing should be considered taboo in general. Plenty of characters in game tend to bend or break the rules, such as Thancred supposedly being a Bard but he uses a weapon not even available in the game (throwing knives), or Y'shtola being a White Mage or Conjurer but having crazy psychic powers (or at least her spells in cutscenes appear like psychic powers).