I am in total agreement that the op suggestion is completely off the wall. (Which is obvious if you read my earlier posts).
I just don't find the third job idea to be applicable either.

PLD/NIN/WAR is probably the least troublesome of what I listed. Being that we might see NIN/PLD/WAR as well or NIN/DNC/WAR (which frankly, would be more powerful than PLD/NIN/WAR in many ways). I only mentioned it because it was the example used.

What of MNK/DRK/WAR or MNK/DRK/DNC?
That would be extremely overpowered in terms of job combos that would be available to other jobs. SAM/DRG/WAR would be powerful but it wouldn't even come close to the MNK combos (Double Attack stacks with Jump), at the very least that combo would be getting TP at a ridiculous rate (as in, more tp than anyone would be able to use fast enough). The cap at which a human could pull off the weaponskills fast enough would hinder the job combo as a whole and make it significantly less effective than the MNK combos.

As for /SMN, it really hasn't been relevant since scholar was released, 3 years ago. Abyssea has nothing to do with that. Even then, it was only subbed on WHM and only for Auto Refresh, to try to bring WHM up to RDM in terms of being able to solo heal a party on birds and the like.

That doesn't really change the other examples either: PUP, COR, BRD, etc.

DRG would be a massive problem due to their af/relic/emp gears and how they can affect the wyvern's sub job and breaths. (Not to mention other balance issues).

As for /BST, I'm honestly suprised how many people don't know about it's sub job capabilities. The way SE figured it was that it wouldn't be as effective as maining BST because of jugpets being BST only. This still doesn't change that any job could and still can sub beastmaster effectively, and charm mobs easily so long as their BST, as a main job would be equal to or higher than the job they are subbing it on. BRD's used to sub it to solo since they could charm enemies with a song and with a job ability, not to mention their naturally high CHR (plus CHR based gears). I've seen a few PLD/BSTs in my day too. Still, it gets largely overlooked for some reason, especially these days. Which would likely change should they add such an idea as second tier sub jobs.

What they really need to do is allow a way to change sub jobs in the field. This seems like a far more balanced approach to the matter. Switching Main Jobs while in the field? No, that's too much, not to mention gear constraints even if it is feasible with mog satchel/sack.