What happens if you don't have a NIN available or if the one needing the fight is the PLD? In XIV you simply wouldn't be able to do it at all. While in XI you had the chance and the option to try to do it differently and adapt to the situation just changing your sub and tweaking your strategy. And too many times it worked with such a wacky setup for me. After all, "Truly viable" is only true until you actually get things done in a different way. (And that's how things like those pesky speed runs were born wasn't it?)
Then it's just about "oh well, yeah, you got the fight done but it would've been easier if you've had the leetest setup!11one" which is ridiculous for obvious reasons.
If you need to get something done it is always better to have options to do it rather than being walled by the limitations. Sometimes it will work and others wont, but even the simple act of trial and error provides emergent content that is not present in a more static game like XIV.
Of course, that doesn't in any way mean you can't actually enjoy having a more linear experience. But from that to actually try to discredit the advantages of having more choices in a game like this (just to excuse the Devs) is simply wrong.
PS > And that's not even counting 1.0's Armory system was even more customizable than XI's since you could take abilities from multiple classes at once without the massive limitations and nerfs present in 2.0. Which brings us back to how pointless the Armory system is right now and how it has become just a vehicle to get you to lvl 30, so you can get your "real" Job and never have to look back at your laughable starting class ever again.