You can, but in practice most people don't. And I can understand why -- it's inconvenient to have to plug in after every short trip, and the short electric range of most PHEV's means you do have to plug in after every short trip.

I plug in my EV around once a week, and it's more convenient than going to the gas station, but I'm not sure I'd want to have to plug it in every time I come home from even a short trip to the supermarket.