I'm curious as to what the official line Square has in regards to this.

Is it because it could be manipulated by botting? I understand not giving out quest rewards but surely there is no harm in quests giving experience for completion.

To explain my experience further;

I had yet to make use of the New Game Plus feature before figured it be nice to level up one of my side jobs my Monk through the Stormblood quest line since it's a bit thematic and I could level and play along with a friend who just started Stormblood.

When I had gotten to stormblood content back in the day I was still doing post HW content so by the time I got there I was already level cap of 70. So It'd have been nice to have played through it as intended with a friend experiencing it the first time and level a side job to boot.

I know many probably aren't too concerned with this since leveling is already so easy but for me it would make the New Game + a better feature overall if you could level alternative jobs with it.

Again if there is a downside to this I am unaware of I apologize. I think NG+ should have quest experience for them. If it's to relive the same story we already have an unending journey to reminisce. There should be some practical incentive to want to NG+ because to be fair Final Fantasy Quests are not exactly engaging enough to play through an expansion with no reward.