I could understand an exhortation to change all four, so that the traction matches and the rear tyres will grip on the corners. I'm still in two minds about that, though as I said I'm getting on so well that I'm inclined to respect the garage's suggestion just to do the front pair.
However, you seem to be suggesting that changing the
rears only is a better option.
This makes absolutely no sense to me, both in logical terms, and in view of my experience last year following the advice from the AutoSocks manufacturer.
http://www.roofbox.co.uk/car-snow-socks/The advice is firmly to fit the things to the
driving wheels. There is additional advice to fit a second pair to the front wheels of a
rear-wheel-drive car to improve steering control. There is no need to fit a pair to the rear wheels of a front-wheel-drive model.
Now obviously that's for AutoSocks, in which you should not be going over 30mph anyway, and they do warn about unstable rear ends if you do anything unwise. As I said, I can see the argument for doing all four feet when fitting proper winter tyres. What I can't see by any stretch of logic is any argument for doing only the rear wheels on a front-wheel-drive car.
Rolfe.