by definition, it is an exploit (a bug or defect you are using to gain an advantage). With that being said, what advantage are you getting? A temporary boon on your MP whenever you want.
Exploits like this are not something that SE will likely ban over (your character does not level or become better from it, e.g. no more exp, gil, or gear), it just makes your life easier... Ok, you can get more exp cause you can waste more MP (and in many ways MP is exp for mages)... So likely this will never get a thumbs up from SE.
As long as people are not abusing it, they will most likely consider it a "We should fix this some time". However, as it gets more and more abused SE will put it as a higher priority to fix. And unless they make a statement about repercussions it doesn't matter much I would guess. But, by simple definition, you are taking advantage of a design flaw.
The most likely result of this is that class ability timers will always restart when switching jobs, even though you don't have to equip them, or those two abilities will work off a shared timer..
Personally I don't have MP problems, so I don't see myself ever using this, but meh, I could see why people like it.
Now, if we started the game and abilities were enabled the way they are now, it would be assumed it was intentional (because we did not see how SE wanted them to be used), however, we can not use that excuse, because we do know, and now it's an issue of if SE will fix it, or acknowledge it as acceptable functionality.