When you have limited MP pools and variances in potency, this is not really the case unless point allocation for classes gets reverted to what it was when the game launched and traits become spread out again.
Actually no. MP variance is irrelevant on the base class so long as the alternative Job carries the innate stat adjustments needed to fill the role. The same can be said for HP values and turning a Lancer's base HP/MP values into something more viable for tanking when it switches to Templar.

Of course the base classes are going to have some limitations on what they can adjust to, they'll be the limits of the base class - pure customization in sacrifice of efficiency.

It sounds like you're letting your personal bad experiences with your guild color your judgement.

There's essentially only two paths here, and the choice is already been made.

Either we make content in which you're stuck to a job and you can't switch roles, in which you're screwed into the the bottleneck of elitism party builds and if you're not the flavor of the patch, you don't get to play.

Or you make the Classes deversifiy into separate roles through the Job system, and you get punished because you don't play the game enough to prepare your class.

Given the ease of leveling the latter is obviously the better solution for our situation. Which is good, seeming that's the way this game is going.

Flexibility does not need a tight leash so much as balancing does.

As far as not wanting to level differing specs due to preference away from playing specific roles - that will depend on how far deviant each class gets from it's baseline functions, and how flexible each group's shell wishes to be.

Hardcore is hardcore. They're going to make you do everything no matter what. Casual groups are more lenient and the Job systems will make things more accessible to them for the more moderate players to be flexible, while retaining their identity. The system is meant to be a compromise, not a total appeasement of either camp.

You can still opt to sit out, but this eases you into other roles without taking you too far from your root identity. The biggest hurtle of which is to get players to identify with their root class more than their specific job. But I have a feeling the gameplay itself will encourage that. Levels are easy. The gearsets can be similar within the same class canopy which makes crafting jobs more focused, even while you're gearing for multiple roles.

And to clarify, this is where I think the game is headed, as all indications seem to point in this direction. How it's reacted to will be up to the players. Speaking for myself, however. I have a positive outlook on it. I think it'll be a fair system that'll do more to bring the community together than many other games out there.