With the launch of Stormblood

[mosaicex]

..There is no OT "role". It's an arbitrary, player-created designation which depends entirely on how your group decides to split up the tanking responsibilities on a given fight...[/i]

Except it is not? If you do any type of end game raids you will realized that the game tries to split the distribution of jobs into the following category

Main(Benefits from taking hits (Shield Swipe, Reprisal, etc.) Tank (PLD,DRK)
Side(Doesn't benefits from taking hits) Tank (WAR)
Main(Throughput) Healer (WHM,AST)
Side(Mitigation) Healer (SCH,AST)
2 Melee DPS (MNK,DRG,NIN)
1 Physical range DPS (BRD,MCH)
1 Caster (BLM, SMN)

Of course this doesn't mean you can't clear the content with every other arrangement, but it would be a complete ignorant statement to declare that the game didn't try to force certain jobs into certain role.

Forgot about T10 Physical and Magical resist adds? How much harder would an average group have it in A10S without Caster or Range LB during progression?

The Limit Break nerf when you take a duplicate of jobs into instance also enforce this theory.

Go ahead and queue for A12S RF and tell me how many time you get a duplicates of healers and tanks and report back.

...if you're actively raiding, then you'll also know that WARs generally tend to be the designated "MT" at the onset of fights because of unchained...

Yes, I do know well enough how broken Unchained and WAR in general is.

But riddle me this,

Can you make the same argument for PLD and DRK?