Umm... because the point of the blades is to concentrate enough downforce on the ice to heat and melt it, allowing the skater to slide along on a cushion of water. You'd have to get some pretty skinny tires for that to be a serious concern.

Personally I've ridden both skinnies and knobbies in the winter. My experience is that the skinnies get me around just fine from point A to point B, but if you actually want to go play in the snow, get knobbies.