It is important for you to have even braking and grip on all 4 wheels.  For example, if you place only 2 winter tires on the front of your front-wheel drive car, the front wheels will have better grip than the rears and could potentially be slowing down fasted on the front wheels than the rear.  Think about it, if your front wheels are slowing down faster than the rear wheels, you are prone to have the rear end slide out sideways and try and pass the front – DONUT!  If you place only winter tires on the rear, you will lose the ability for the less grippier tires in the front to effectively steer the vehicle and you will find yourself going straight on a sharp turn – OFF ROAD!

Don’t take my word for it, watch these six short videos that appear on the bottom of the link.