>"I dont get" Ultra light winter back packing. I have to carry so much extra gear to keep warm I just dont really get trying to lighten that up.
For sure, ultralight winter backpacking means a very different thing than the other 3 seasons. The UL threshold goes up accordingly. That doesn't mean that you can't still strive to watch weight and carry less, without being stupid about it. I can keep my winter base weight to ~15lbs and be good to 0*F.
>What advantages does a backpack have over a sled in deep snow?
I've never pulled a sled or pulk, so take this FWIW. They may just invite you to carry more stuff because it can hold it. Every pulk I see is laden with pounds and pounds and pounds of stuff. Even in winter, I like to keep my kit as minimal as possible.