I agree that the idea of using an electric system as an intermediary is not a great one, though I know this has been done in bicycles with efficiencies as great as 80% (i.e. pedal into a dynamo that powers a hub motor in the back wheel, rather than using a chain).
I have often thought that the ideal would be to combine the engine and electric motor, so that the motor could play a triple role as a starter motor, dynamo, and to increase peak power output during take-off or aerobatic maneuvers. This way, the engine would feed power directly into the propeller without needing to convert it to electrical energy and back again. You could operate the petrol engine at 100% power throughout the cruise, and optimise it for efficiency at this setting.
Some of the newer lithium polymer batteries have astonishing power densities. For example, a battery weighing a kilogram might be able to supply over 30 volts and 200 amps for a minute or so - about 10 horsepower. Brushless motors have a very good power to weight ratio too - my 3 horsepower motor weighs about 700g. So you can get large transient amounts of additional power with a relatively modest increase in weight.
This would be more important for cars than for aircraft though, because the difference between cruise and peak power output is greater for cars.