I live in Finland where it's enforced by law to use winter tyres during December, Janyary and February. Studded tyres can be used from November to March, traction tyres year-round (although it's not recommended because of the decreased fuel economy). During this period any cars that don't have winter tyres on can't be driven.
I've driven with summer tyres when there was snow and ice, and after swapping to studded tires, the feeling of control increased tenfold. Almost all skidding went away.
My BMW's tyres are so wide it'd be no use to have studded tyres because they wouldn't sink deep enough for the studs to grip ice/road, so I have traction tyres instead. Haven't driven with them yet because I've only had the car for a month or so, but winter is coming...
So I really recommend swapping winter tyres on for the winter months when there's slippy snow and ice on the ground, unless you particularly fancy finding yourself and your car from the bottom of some ditch.
Oh, diff-lock and winter tyres in a RWD vehicle makes it perform almost as well as a FWD when e.g. trying to get up a slippery slope.