Oh this is something for the long haul, not something to be implemented all at once; but I feel the problem with FFXI was (with jobs that is) was that they didn't really KNOW what jobs were going to be released until AFTER the initial game was released. Like Dancer and Scholar, they were both really out of the blue jobs, almost felt like they were just pulling names out of a hat.

If you have a clear set in stone (or pretty sturdy mud) path then you can make a balance of jobs that offer unique abilities and spells to the game and parties. Also like I said you can plan content and monsters that are easier targets with certain job setups (monsters that are only weak to blunt, or only weak to magic or only weak to piercing etc etc) The usefulness of a job isn't what is has, it's what it can use it on effectively. If that makes sense.

As far as gear goes, since it takes them 6 months to make the gear, having a rough idea on your future jobs makes a lot of sense. Also I would expect to see the current Classes → Jobs we have now to have just about all their own specific gear getting its final touches.

Also while sitting here (working on my own RPG project) I thought of an idea for Land classes basically it would work the same way except to help narrow down the "search" for certain items they can get various job upgrades to help

Botanist → Herbologist (increases chances to find small plants "Moko Grass" and the like while decreasing the rate of "lumber")
Botanist → Arborologist (same as above just visa-versa effects)
Miner → Gemologist (increases rate of finding precious metals while decreasing all others)
Miner → Paleontologist (increases rate of finding 'bone' materials while decreasing all others)
Miner → Geologist (increases the rate of finding raw ore material; iron and the like while decreases all others)
Fisher → Marine Biologist (increases rate of finding large fish)
Fisher → Limnologist (increases rate of finding small fish)

I don't really play as a Land class so I'm sorry if I have not covered all the areas (I've only played Miner a little bit), but something like that would be neat I think, no? Of course it wouldn't make it where you ONLY find those said items but it would increase the rate of finding them.