Personally I'm not a fan of long term ambiguous grinds. With a system like this I would either feel somewhat forced to play a max level job at all times, like any time I spent leveling another job would be time wasted (especially if the rewards increase your power somehow), or I could choose to not care in which case it may as well not exist. When you put content in a game you want people to care about it, that's why it's there, and if they care about this it feels imposing and limits their freedom to do whatever they want because it never ends.

I don't find this kind of thing rewarding either because you don't have a clear path or goal, you just get a bonus sometimes. You don't really earn anything with a system like this you just derp around for a while until oh hey free stuff, cool I guess, whatever. A better sense of reward comes from effort.

Besides if it's tied to exp, roulettes and dungeons are probably the best way to go anyway so I'm not sure what this would accomplish that additional ways to spend tomes would not, and it certainly wouldn't help if you're bored of them. What else would you do to increase your rank, fates? Hunts? Beast tribe dailies? Crafting and gathering? They have their own rewards. Side quests? They give garbage exp. (I would like to see a better reason to do side quests though.)

You could also look at gil as a similar system to this already - you get it from just about everything and can buy all kinds of stuff with it, even a house. Why would we need something new that fills a similar role? The only thing unique this new system would bring is an arbitrary number denoting your rank - any of the rewards could be obtained by other means.

Finally there is a lot to be said for being able to finish something, as others have said here already.

Does FFXIV need a system like this? Definitely not, there's plenty to do with more being added all the time, and plenty of rewards to go with them. Would it benefit from it? I think it would actually be more harmful than good.