Personally, I'd agree with a way to stop experience point gain as an option and/or creating a level cap for your team once you've achieved or surpassed that level (but didn't gain exp according to your actual level). I don't think it would break the game one way or another, but it would enhance the ability to work with newer or less experienced players without having to create another character.

Say, if I wanted to be a WHM for people new to the game, but didn't want to be able to cast with my full store MP thereby exhibiting MP management for other mages or helping the tanks learn hate control (by casting on a similar level and at a similar rate of a WHM closer to their level), I'd be able to adjust my level to suit theirs. Otherwise, it'd be like having a overleveled WHM who could just unleash a curing spell at anytime to adjust for any mistakes made.

Like Carbuncle said and Tristan somewhat suggested, Bravely Default allowed for a couple of different ways to stop EXP gain to enhance the gaming experience. You could simply turn off EXP gain or you could equip an item that converted experience points to pq (gil). You could also turn off random mob encounters, but I think that would potentially take away from the game itself here.