[fan]

I don't believe the engine or the generator are limited to that optimum rpm.
If the battery is discharged and the driver keeps flooring it, the gas engine could be revved up(maximum power) so that the generator produces enough electricity to keep up with driver's will. So, it shouldn't happen for one to not be able to get through hills if the battery is depleted.
But this would be a highly undesired case. Stress the battery and increase fuel consumption.
If a diesel engine-generator setup works for a locomotive it would work for a Volt too. The engine of a diesel locomotive is surely not limited.
However, I still don't get it why some power cannot be sent directly to the wheels from the generator when ICE is running. Charging-descharging decreases life of the battery.
This could be a good PR thing too. Volt works both as a paralel hybrid(without mechanical transmission (gas engine-generator-emotors)) and mostly as a series hybrid.
So, it does what Prius does and even more than that!

Edit: And having a direct path from generator to e-motors, would allow sending as much power is needed to the wheels when battery would be discharged. This would allow designing a battery that doesn't have to cope with extreme conditions.