A class is a basic root for weapon related skills/spells/passives. Jobs on the other hand have the specialized abilities which make them unique. I don't see why it's hard to imagine a class having more than 1 job. Jobs already have set requirements (in the case of White Mage, 15 GLD along with a 30 CNJ), so a SMN could require a 15 THM, while a Scholar a 15 CNJ. (Or ARC, PUG or whatever)

This doesn't even get into how Jobs have the skills of a second class on top of the required to level 15 one.