I'm in SE MI, so I just got a little of that storm. You can run the stockers all winter, but when I talked to the people at Bridgestone they said the tire compound starts to "freeze" at 40 degrees, and gets worse as temps drop. I have a steep driveway and I have to deal with soccer moms flying up and down my side street in SUVs dropping their kids off at school in the morning, so after getting stuck in my driveway a few times I broke down and got snow tires. They will go on the car again tonight. After the change I stopped worrying about leaving enough stopping distance and started worrying about getting rear ended. The best way I can describe the difference between the stockers and winter tires is it's like taking off roller skates and putting on shoes. You can get around fine on roller skates if you know what you are doing, but I'm still way more comfortable wearing shoes. But I suck at roller skating and have only gone once when I didn't have to, so take the analogy for what it's worth. But the big thing is the tires get worse the colder it gets.
Still though, I see winter tires in winter like slicks at the drag strip. You don't need them, but to get the most out of your car you should get them. All winter long guys in SUVs try to pass me on those stop lights where it narrows to 1 lane right after the light, and I get an adrenaline kick every time they can't.