Funny how the cost/enforcement question comes up.
In Germany, it's the law. Sure, the police aren't going to walk around streets checking. But if you are stopped for something else, or if you are involved into an accident, it becomes a fine. And then your insurance might choose to deny coverage to the damages.
So despite many parts of Germany having oceanic, tame winters, people still put them on. Expensive ? Yes, but that's just part of the cost of driving a car. Too bad.