In Ontario we are blessed with a full range of weather. This year we've had several bouts of freezing rain, and more snow than in over a decade. It seems some drivers are tiring of properly clearing their cars of ice and snow.
In Ontario it's an offense under the Highway Traffic Act not to clear off your windows at the front, back and sides. You aren't required to clear off the top of the car if you can't reach it, but if you can you will face insurance implications in the event you cause an accident by blinding cars around you when it blows or slides off in chunks.
When you drive with your car covered in ice and snow you are posing danger to yourself and others around you. Your impaired view means you could easily miss other cars or pedestrians in your path. You may also not see emergency vehicles behind or beside you. Your lane changes and turning will also be unsafe.