It hurts to be maxed out on energy and have absolutely nothing to spend it on. It hurts twice as much to be maxed out on energy and hit a big level where it recharges your energy entirely and it does absolutely nothing. Convert for BP, perhaps. Or spend on roulette spins. Or for kobans (very heavy exchange rate there). Something, anything, but energy unspent and unspendable means there's less reason to come back and muck with anything in the game.