I would quite upset if it was decided that you could only level one class/job per character.

You can somewhat mimic the concept, though, by restricting yourself to one class; the option is entirely there. Technically, you don't need to set cross class abilities, but not having them would likely hamper your role and flexibility, and would certainly decrease the chances of playing with serious players. But if that didn't matter much to you, simply stick to one class. But that should be a play style option, not a requirement. The one thing I love about XI is that I can play on BLM or BLU or RDM, and my roles can vary. Too many BLM? No problem, I'll switch to something that will be more beneficial, and I wouldn't have to ensure that I've unlocked a certain area on an alt (which back then was a lot more stressful than it is today, comparatively speaking). It's why XI has been the only MMO I've settled with, and the main reason I was put off giving WoW a real try.

What really made it more amazing for me - having the ability to switch jobs - wasn't in the jobs I could "main" but the support roles I could acquire and the possibilities that were unlocked. RDM didn't usually sub NIN as part of it's main role, but doing so opened up a new world. XIV lets you set a variety of support abilities that you could unlock by levelling the respective job. It's both flexible and restrictive, with some cases being more one than the other, but it lets you add the role you are playing. The thing I don't like about it, is that jobs are restricted from certain classes and classes themselves aren't really good enough to be used on their own, but that's a different argument for a different topic.

From my point of view, having the ability to customise your character to play in a way that you want, regardless of whether you have 1 or 5 "main" roles, is what makes being able to swap classes/jobs that more appealing. Take that out, I could enjoy the game a little but no where near as much as XI; and as it's eventual successor, XIV needs to take those things that were very well received and build upon them.