I use New Game+ so I can follow along with a couple friends as they do MSQ, but when there are enemies to fight in the open world I kinda just stand around and let them handle it. Maybe it would be nice if New Game+ itself allowed you to level-down to the level of the quest you're doing, that would be something. Outside of that though, I don't think the feature would get a whole lot of use -- not to mention, many zones have quite a level range depending where you are on the map. What is the "level" of North Shroud, or Southern Thanalan? Or are you suggesting you just manually set your own level?