What are some alternative benefits to electric propulsion?
For aircraft, none.

Hybrid drive trains are used because:


They offer, rather limited, efficiency improvement by being able to operate the engine closer to its optimal conditions.

Because spark-ignition engines are more efficient at higher power settings, the hybrid can get advantage by having smaller engine, running it at (relatively) high power or not at all and cover the variations in required power using the electric engine.

Cars accelerate and stop a lot and often cruise at small fraction of power, so they can make good use of this.

The advantage only applies to spark-ignition, i.e. gasoline, engines. Compression-ignition (diesel) engines have more uniform efficiency over their operating range, so there is no or only negligible advantage. Which is part of the reason you don't see hybrid diesel cars.
Batteries can be pre-charged with cheaper mains power. This is used in the hybrid cars, but it obviously requires lots of heavy batteries.
Electric motors can provide high torque from standstill. Piston engines can only provide limited torque from standstill and it stresses the clutch heavily.

This is particularly important for train engines. Train wheels have bearings lubricated by oil pumps attached to the axles that only operate when the train is moving, so when the train is stopped, the bearings are not lubricated and the force required to get it moving is much higher then when moving.
At high power outputs, electric generator and motor are simpler and more reliable than a gearbox and clutch. For big train engines and ships, mechanical clutch is out of question. Hydraulic one, which is a turbopump and turbine in oil, can be used, but that still needs variable gearbox. The electric drive-train acts as almost ideal continuously variable transmission, being able to deliver almost constant power over wide range or rotational speeds.


In either case the hybrid drive-train is heavier than mechanical one.

Now for aircraft:


The engine is run at constant speed and power setting, which is relatively high (usually cruise setting is 75% for piston and even higher for turbine planes). So there is no room to design a hybrid drive train to run the engine closer to optimal.
Weight is critical for aircraft, so the weight of electric generator and motor is a show-stopper there. For cars adding weight causes smaller increase in drag, so its less of a concern and trains even need to be heavy to have enough adhesion for their traction.
Since weight is critical, aircraft can't afford to take much batteries that could be pre-charged with cheaper mains electricity, the other benefit hybrid cars have.
Propellers need almost no force to start spinning and are operated at narrow range of RPM, so the ability of electric motor to provide torque from standstill and across wide range of RPM is of no use to aircraft.


So none of the benefits for which hybrid drive trains are used in other vehicles is applicable to aircraft, so there is no point in building one.