In an ideal world I think you're right, but practice has shown that many buyers of plug-in hybrids never actually plug them in.
This was especially when they are bought for large fleets - which makes some sense to me, after all, when it is your companies car, not yours, and its on the company's dime when you either fill up the gas once a week or charge it every night, why do the thing that is more annoying?
it feels like it should be easier to plug in a few times a week rather than drive to a gas station, stop the car, exit the car, insert card, enter pin/zip code, open gas cap, remove nozzle, insert nozzle, pump, wait for pumping, remove and return nozzle, close gas cap, take receipt (optional at some stations), return to car, start car, and leave. yes, you have to stop and start the car when plugging in at home, but you already have to do those things, whereas driving to a gas station is additional stopping and starting.
This was especially when they are bought for large fleets - which makes some sense to me, after all, when it is your companies car, not yours, and its on the company's dime when you either fill up the gas once a week or charge it every night, why do the thing that is more annoying?