I think the OPs point was not the lack of xp, but that outside of dungeon treadmill and roulettes, most content give too little xp. Pixies being an notable exception. FATEs and subquests (who are a one-and-done deal) give a meager 1-2%, though to be fair the former can be combined with the weekly Challenge Log. They could theoretically increase quadruple subquest xp and it would have no effect on any roulette or dungeon treadmill due to its one-and-done deal, especially if you level all jobs on one character.