Advice from Michelin (who obviously want to sell more tyres, but it makes sense)
Cold weather tyres should be fitted on all four wheels
With cold weather tyres only fitted on the drive axle, traction is improved, but this is not enough to ensure good performance and optimal safety. With cold weather tyres only on the front axle, you risk losing grip on the rear tyres and going into a spin. With cold weather tyres only on the rear axle, you could lose grip on the front tyres and understeer instead of taking the corner.
http://www.honestjohn.co.uk/faq/winter-tyres/
From Dunlop:
10. I can just fit Winter tyres to the driven wheels?
We strongly recommend you fit a complete set of winter tyres. This increases vehicle traction, stability , braking and cornering, ensuring your vehicle is well balanced and has a stable footing on all road surfaces. If your car is front-wheel drive, and winter tyres are fitted only at the front, there is a greater risk of losing control of the vehicle. If your car is rear-wheel drive, and winter tyres are fitted only at the rear, there is a greater risk of losing control when cornering. Finally, if winter tyres are fitted only on the non-driving wheel axle it will be very difficult to move on ice or snow.
and Finally a video test (again from Michelin)
http://www.tyrereviews.co.uk/Article/Video-Can-you-fit-just-2-winter-tyres.htm
Its not just about stopping and starting in a straight line. If you have to brake when cornering your back end can step out and you may as well not have bothered with the 2 front ones.
Additionally - Most insurers want you to tell them you are fitting winter tyres. Good luck telling them you are only doing 2
EDIT - its also illegal in some countries for a reason:
http://www.tyrereviews.co.uk/Article/Video-Can-you-fit-just-2-winter-tyres.htm