You still need enough power to make a late go-around at the destination, in case of problems due to weather on final approach, arrival spacing, or a problem on the runway. I've been on planes that had to go around from low altitude three times, so it's not even that rare. Either the engine needs to be large enough to generate full take-off power, or you need a large margin of battery power that will never be used on an uneventful flight.