moana,
If you use the search feature, and look for "snow tires", you will find a wealth of info about winterizing your smart. In short, based on what I've read (in South Carolina, we have to import snow tires, and even then you get some strange looks), you may want to consider purchasing a spare set of Pure wheels (the steel ones), and get snows for them. The stock tire may handle light stuff, but you don't want to go through a snowy winter on them.
As to the durability of your tires, good question, and one you will get many answers to. In order to maximize the lifespan of your tires, you will not merely rotate them, but have them dis-mounted and turned around on the rims, since the tires sit on the road at an angle, and the insides will wear faster than the outsides. This does cause the tires to wear faster than "normal" (the slanted tire thing), but provides more safety and stability.
I just replaced wheels and tires (voluntary - I wanted something different from stock), and at 12,000, the original tires are showing wear on the insides (I hadn't rotated them yet), but still have many miles left in them.
Cheers!