One of the most interesting features of the Z1 is its unusual doors. These doors retract vertically down into the car's body instead of swinging outward or upward. The inspiration for these doors came from more traditional roadsters which often feature removable metal or cloth doors. Because removable doors did not fit within BMW's design goals, the retractable doors were installed instead.
Because the body, with its high sills, offers crash protection independent of the doors, the vehicle may be legally and safely driven with the doors up or down, although they are not legal in the U.S.
The windows may be operated independently of the doors, although they do retract automatically if the door is lowered. Both the window and door are driven by electric motors through toothed rubber belts and may be moved manually in an emergency.
What is your point. Batteries go dead plain a simple. Your battery dies in one of those cars and your door is stuck open/close until you charge it up or replace it.
I do not even want to think of how expensive that design is. I do wonder how much weight/steel they have to add to the roof to make up for the lost structural support in the middle of the car.