I think that if they did enable xp for New Game+, they'd nerf the amount it gave so that it wouldn't be too easy (or exploitable) a method to level. The MSQ quests weren't originally intended to be repeatable... the xp they gave was to help someone keep their level up as they went through on their main job, and leveling alt jobs was always meant to be done in other ways.

And as Fynlar says, leveling is already extremely easy in this game as it is... with dungeons giving about a third of a level even in the 70s, with +30% xp accessories for 1-70, PotD and HoH being even faster options, I don't see a need for an even easier option than what we already have access to.