Honestly, I don't see an issue if some jobs are better in certain high-end fights?

If PF locks out NIN, just go to the MB and buy a VPR weapon, I can see it being an issue in other MMOs where you're locked to one class on your character, but you can swap to any job in this game.

If you want to prog harder content where people are more likely to lock out jobs, sometimes you have to play jobs you may not like for a better prog experience, ask all the BLM mains who had to play RDM/SMN for prog.