You can actually get aftermarket windows for the Twizy. They are made of plastic and held on by velcro.
I think they cost around 300 pounds.
Search on YouTube for "Twizy windows" to see a demo.
Supposedly they keep the interior (including driver and passenger/shopping/dog) dry and require no permanent modification of the vehicle.
Personally, if I had a Twizy, I wouldn't like the idea of driving around and leaving my car (if you can call it a car) parked with no windows. Nor would I want to leave the car with the windows attached only with velcro - where I live they wouldn't be there when I returned to the car. Taking the windows with you when you pop into Tesco would also be quite impractical
I understand the arguments about keeping costs down, but if Renault can offer doors as an optional extra then surely they could offer non-velcro i.e. permanently-attached plastic windows as well?
Still, I'm tempted to go to my local Renault dealer and take a Twizy for a spin. Looks like fun!
Here's a nice little review of the Twizy by Kryten if anyone's interested