Interesting. I've always thought of variable pitch propellers as being more efficient by allowing the engine to stay at it's peak horse power. For a piston engine this is obviously at the peak of the nice curve you'll see on any dyno run. A fixed pitch propeller sized for climb would allow the engine to run above this speed in cruise and vise-versa if it were pitched for cruise.
Now an electric motor has maximum torque at 0rpm (stalled) and practically no torque at it's no load speed. The relationship in-between is practically linear. So long as the motor was matched and / or geared so that during all stages of flight the RPM stayed within a reasonable band I don't see what the advantage is. Maybe some second order effects of efficiency at lower speeds perhaps.
Edit: Actually, thinking again, it's the exact same reason that cruising at the lowest possible RPM for the required power output in a piston engine is typically the most efficient. Just less friction. Still think it'd be a smaller effect given we're talking roller bearings rather than plain bearings and piston rings.