Cots get you off the cold ground, but let air circulate freely. You sleeping bag will compress below you, which means you will get quite cold.

I suppose you could use a cot in the winter, but you would also need to haul adaquate insulate to use with the cot. The cheapest option would be closed cell foam such as the basic "blue" pad sold pretty much everywhere. In really cold weather you might need 2-3 for adequate insulation. The other option would be to use an "under quilt" like used by winter time hammock users. Basically you suspend a quilt tightly against the bottom side of the cot fabric so the insulation doesn't get compressed.