The way I see it, as soon as you turn off, or cycle, you have to slow down due to drag. Then you have to re-start, which is inefficient, then accelerate, which is less efficient, and then the cycle repeats itself.
I'll try and answer from a bunch of different angles.
An engine sucks a lot of fuel just to keep spinning. The less time the engine is on, the less fuel is lost this way.
Imagine two hills next to each other, of the same height. I start at the top of the first hill, heading to just below the top of the other hill. I try several different methods:
1) If I just leave the engine off, and coast down, I'll end up almost at the top of the other hill. I'm not at the top of the other hill because of aero and tire losses. To this point, zero fuel used.
2) If I do the same thing in neutral, I use the amount of fuel that it took to keep the (damn) engine idling the whole time.
3) If I keep the engine in gear, I use more fuel because I'm compression braking the whole time, and then have to make up that lost energy by using more fuel going up the second hill.
4) If I try and keep a constant speed down and up the hill, then I used extra fuel to accelerate to that speed, pressed the brakes to keep from going too fast, and then had to use more gas going up the other hill to make up for both the brake losses and the motor losses.
#4 is how most people drive, because it is safer, and usually faster because you accelerate to the legal limit faster. But it isn't more efficient - in fact, to the extent that the driving differs from #1, it will require extra energy. Now, #1 goes really slow and then fast at the bottom and then slow again, and it's really annoying to everyone to be speeding up and slowing down all the time. But the range extender in the Volt could cycle on and off all it wants, since it is disconnected from the wheels of the car.
The point is to analyze from an energy perspective.
Cruising on level ground doesn't require a lot of power. Maybe 10hp to overcome the aero/tire losses? Assume that the peak efficiency point of the engine is 70hp. That means that running it at 10hp is as if we just left the engine at idle, wasting gas. (For the moment, let's ignore the energy used to start a warm engine) If we cycle the engine on for 10 seconds and run at 70hp, and then off for 60 seconds, the average power is 10hp. But the engine was off for most of the time.
Now running for 10 seconds at 70hp would speed up the car a lot, and coasting for 60 seconds would slow down the car a lot, so this is impractical.
But in the Volt, the battery is absorbing the power difference between the generator and motor, and the car is maintaining a constant speed. It would be pretty reasonable to run the ICE for 100 seconds (the ICE generating 30hp and the electric motor using a constant 10hp) and then turn the ICE off for 200 seconds (the electric motor using a constant 10hp) - the battery SOC would only change by 2.5% each cycle
As compared to the generator running at 10hp constantly, I would expect the fuel economy in charge-sustaining mode to increase from 50mpg to ~80mpg with a 1/3 ICE duty cycle.