Lots of thoughtful characterizations of the use case here for the RR-hybrid system. If you want to observe the benefits in the form of a real-life implementation then go drive a first-generation (no plug in for the battery) Toyota Prius for a few days. The engine kicks in when needed to top off the battery, accelerate aggressively (TOGA Power) or to regeneratively charge the battery (think about a descent from 20,000 feet during which your only speed brake is spinning prop recharging the battery. As the system evolves the engine will be used less and less until the energy density of batteries is sufficient to cover the entire mission. Pretty excellent research platform IMO.