"New game plus - quests will have no rewards..."

[Nemmar]

Because people will use it for spamming. It's that simple. Bots, anyone?

Do you see flocks of bots in WoW or any other major mmo? Do they not have quest rewards? Bots are an issue that squeenix has to solve on their back end. It's not something to be solved through game design.

As you have said, if it's not the best Xp/minute people won't spam it. They will do it for the story and a bit of XP or gil or whatever they would add. They could spam it, but as long as it's not a very profitable option, it's unlikely they would.