Quote Originally Posted by ADarklore View Post
Since I'd be using the boost on a job that I would never want to play in the first place, I wouldn't miss not running the class questline, yet it would secure me XP boost to the lower level classes that I DO want to experience leveling
All but one of those classes will automatically get the XP boost anyway, regardless of whether you buy the level boost or not. So you'd be paying just to get the boost on one more class. (Personally, I'd find that benefit too minor to make the purchase worthwhile, but there's nothing actually detrimental about it so long as you weren't going to play the selected class anyway.)

And if you do have the xp boost on all the classes you're playing, you'll have to be careful about alternating which you use for content. If you try doing all the main scenario quests on a class with that XP boost, you could get pretty far outleveled for the quests you're doing. Since you're planning on leveling multiple classes concurrently anyway, you can avoid that problem by doing a few quests with one class, then the next few quests with a different one, and back and forth like that. You just need to give a bit of thought to which you currently want XP on when deciding which to play.