Until 1.2, Thaumaturge was more of a shadow priest / ritualist DoT class who uses Astral/Umbral magic (and tended to be the stronger healer and saboteur), and Conjurer uses all six elements as a synergist elementalist theme of caster (and tended, outside of certain levels, to be the stronger caster DPS). Although most relevant lore was removed and then replaced with ARR, traces still remain from prior to Yoshida's decision to scrap these entire design concepts just so he'd have a smoother base for Black Mage and White Mage respectively.
I, too, actually really like the idea of split jobs—Conjurer > Geomancer, Rogue > Thief, Archer > Ranger, etc, if only because the bastardized legacy of the armory system that we have now might well otherwise excuse the refusal to ever, say, play around with the elemental aspects promised by a Conjurer or inherent in early White Mage play (lest they replace every light-side elemental spell with simply Astral magic) simply because there's "already a job/weapon for that". Such systematic refusal does nothing but waste assets and drive future designs into corners. And what's the alternative, to give Geomancers bells and Ranger "rifles" (removing every long-ish gun from machinists), and Thieves... sickles? For what advantage? You could require jobs from the same class to be leveled separately, acting as a new class that requires the former at level x to start, rather than a mask over the class. Every single skill can be adjusted for each branch job separately, even skills prior to the level at which the job is acquired, so I really don't understand what's so wrong about classes that Yoshida feels the need to remove them, just to make it so that we all start, from the very moment we pick up a rusty shortsword (usually from the gladiator guild), as a member of a traditional monarchical vanguard of waning importance and loyalties and vague connection to Holy magics, despite learning from anyone and everyone with a sword (generally gladiators) for the first 30 levels and having zero connection to Holy Magics for the first 50 to 58 levels.