Personally, I won't drive a car without 4 winter tires. It's just cheap insurance. You may be the best driver but you have to be able to react to other people's mistakes (being able to brake/stop when some jack*** cuts in front of you and stops for eg.). The ability to stop just a little sooner can be the difference between avoiding a crash or having a claim.
I won't even rent a car in winter since they don't come with winter tires. I'd rather take the TTC if I had to.
You just reminded me, I have to start shopping for my GF's winters. Time to start looking.