I don't have a winter tarp, but have been out a few times in the conditions of wind rain and fog. Using a hammock cover/sock has kept me dry and warm. The advantage to the winter tarp I think would be to have a weather protected area to store gear and cook which I can do with the tarp I already have. Why clutter up ones pack with extra's that are "nice" but certainly not neccessary. Could see more of a use for it in drifting snow conditions perhaps, but here on the West Coast of Vancouver Island thats not an issue, its the rain.
What I lack in knowledge I MORE than make up for with opinions.