When going to special traction tires like snow, winter, or race tires, it can actually be quite dangerous to only replace two. (Even worse to only replace one!)
If the fronts have a significantly higher traction than the rears, it's nearly guaranteed that you'll swap ends under hard braking. It becomes easy to put so much braking power on the fronts, that the rear end loses what little traction it had and will swing out around you. Not to mention the extreme tendency to snap oversteer suddenly during cornering. In addition, your ABS system may not cope well with the radically different traction profiles of the front and back, giving you poor overall braking force.
The difference between a Blizzak tire and an all-season tire (like the stock RE92) in the snow is night and day. I have to agree with Tirerack's and Bridgestone's recommendations, it is not a marketing scam, it's a safety issue.
Replace all four. Better yet, buy a set of ultra cheap rims for your 4 new winter tires, and use your good rims for your summer tires - then just swap rims when the seasons change.
