I used the multiple turn in leves for leveling when I took my crafts to 50 (gathering to 50 at the same time). Those who say that this is quick are not representing the whole picture. In order to do the multiple turn in, you need HQ. When you're leveling up, getting HQ is not easy and needs HQ mats. This took a lot of time to gather and then to craft for the turn in. Of course, if you're just spending RMT gil, there's no need to do the gathering.

Leves are once again an important thing as I now obtain the untradeable items needed to craft the Lucis turn in items (obtained through leves or bought with GC currency). To get one crafting job ready for Lucis turn in takes ~ 30-40 leves worth of activity depending on the RNG (leves don't always offer the item as reward).