well, what would you want energy to spend on? random quests without any rewards? nobody would really do those and the workload necessary would be just as large. on the other hand, if they were to provide rewards of any sorts, they'd most likely loose a possible source of income, since you can get your cash by your harem, but getting new girls would only be possible by higher combativity rate or by cashing. and no way would they reduce the only real value in cashing for players.
don't get me wrong, i also would like to see an option to spend energy on, but i doubt we'll ever see anything, at least, any time soon. we're just not as attractive anymore.