Actually, Red Mage could wear more than leather armor, they COULD wear scale mail, equip up to kite shields, and the AF armor had plate pauldrons, bracers, and greaves, even if they were not described as such, and Blue Mage could wear similar armor sets. So ffxi actually had TWO mage jobs that could wear up to medium weight armor. Even in Lord of the Rings, Sauron was depicted as wearing full-plate armor, despite being a sorcerer and "Necromacer", and I don't think I need to describe the Witch-king. So there are a few precedents where mages can wear armor heavier than simple robes.