Just leveling exp won't be enough. It has to be something worth of value for everyone who maxed out their characters - something like high amounts of gil / tomes / popular renewable currency that can be used anywhere no matter ilvl difficulty.
A fully maxed out player might be doing their roulettes for tomes (and that's the only thing that matters now if you want to farm tomes for relic weapon...) and the side boss gives nothing of value except time wasted for that goal because it becomes "optional".
