I would have liked to have seen this implemented long, long ago; even at the level 75 cap.
How about having a third job that caps at half your support job level, 1/4 of your main job level?
I.E. PLD90/NIN45/WAR22 or COR90/RNG45/NIN22, or even BRD90/WHM45/BLM22 (so you could both backup heal and ES songs when needed)
I don't think there are too many JA or spells or the like that would break the game balance if it was implemented with caution. Like nerfing Provoke to grab less hate if WAR is your tertiary job or making utsu: ichi only grant 2 shadows, etc. Even at level 99 you would be capped at 99/49/24, so you wouldn't get some of the nice level 25 abilities that might break game mechanics, though none really come to mind at the moment.
I think it could be a great way to build some more versatility into our characters.
I do realize, though, that it would probably take far more effort to implement than SE is willing to invest. It could, and probably would, break a few elements of balance we'd probably want to preserve. But hey, it's just an idea!