Winter tires are not snow tires. Winter tires are cold season tires. The temperature of the road is different to the air temperature, so even if there are 10C in the air the road can be 7C or lower. And even on dry surfaces the winter tires grip better than summer tires.
And if you consider that 15" wheels and tires are a lot cheaper than brabus 17" wheels and tires, you can see that throughout the life of the car you actually save money. I know it can be a bit inconvenient when having to pay for a full set of wheels but trust me, they pay off in wear saved on the more expensive ones, and god forbid accidents avoided.
A crash at 20mph can be the end of a forfour, and that's the speed the summer tire equipped car has when the winter tire equipped one has already stopped, and it's a 30mph test!