I use snbb year round and the bees come out of winter in great shape. They have a 3/8" high x 3/4" wide entrance on either side of the landing board and I have yet to find a dead mouse or signs of one in any hives. My colonies are on metal frames @ 6" off the ground.
This is 2011 winter with @ 18" of snow.
I think your location might of had something to do with your luck. Here in Michigan in a normal winter we would never get away without mouse guards. The mice come in after the bee's cluster and chew through all the bottom frames. The bee's are only inches away but dont break cluster.