Why would you ever want a Paladin and a Dark Knight to stem from the same class? Why does that seem to be a go-to example? Forget their FF lore for a second -- in terms of gameplay, they're pretty extreme opposites. I can't think of many skills I'd be happy with them sharing.

I have plenty to say against classing getting more than one job, but that seems to be a second argument. And it's not what OP was getting at.

So here's my suggestion. It's simple enough that it can actually be conceivably done without changing the way anything works:
  • Balance the stats between jobs and their respective classes. Jobs should have more specialized bonuses (specialized, not stronger) while classes have a more even spread. But balanced means balanced in terms of application, not that they have a mathematically equal sum. These spread out stats have to actually give the classes some benefit rather than just making them less competent and their job's role.
  • Increase the number of job-earned abilities. I'll use 10 as an example here, though the actual number would have to be tweaked to find a good balance.
  • Give jobs a small number of free slots to take skills from its two partner-classes. We'll say 5. Jobs retain use of all of their class skills.
  • Since jobs have 15 abilities outside what they inherit from their class (10 job abilities, 5 free slots), give classes 15-20 free slots. Again, I don't care about the actual numbers, but just gernally. Give them a bunch. Enough to make up for the fact that job-specific skills are stronger.
  • Obviously, classes would need enough abilities that were unique to them and their job in order to stop all classes basically being able to achieve the same or similar ability spread.

Yes, I'm fully aware that that's basically how it's set up right now.. my point is this: The idea that jobs are highly-specialized and incredibly powerful within their specific role, while classes are far more versatile and sport increased flexibility in solo or low-man situations is fine. What is bad about it is simply what everyone's already pointed out -- that isn't how it works.

So why not just push what we have a little further in either direction instead of going crazy with overhauls? I'm very optimistic that, come ARR, we will see a... manifestation of this sentiment, in one form or another. Make jobs even more specialized than they were -- they should stand very distinct from their classes once they're fully developed (through the use of more than a measly 5 unique skills). But actually make classes as versatile as you promised us they are. Hell, in completing the class-specific quest why not award me a bonus when I'm on that class without a soul crystal equipped and without a party?

Make me WANT to solo without my job on, SE.