Regarding TomD's comment:

"My point is this: sure you might be able to get away with a tarp or light tent under ideal winter conditions.,,. But, if things go bad, unless you have the right skills and the right gear". etc.

Here's a newspaper article about crossing the Juneau Icecap in first week of April with Megamid:
http://www.juneauempire.com/stories/051001/Out_Icefield.html

Last weekend of February, a highly routine trip, using Megamid as a matter of course, in coastal mountains near Vancouver (See page 14).

http://www.alpineclubofcanada.ca/calgary/chinook/Chinook-Apr01.pdf

Etc. ..There are plenty of other examples. The founder of Mountainsmith packs later developed Kifaru tipis essentially as winter shelters in Colorado. (They are in effect, over-sized Megamids). The book "AMC Guide To Winter Camping" (1991) had extensive recommendations regarding similar tents. Book had a northern New England focus. Etc......

The idea that a properly designed, fully enclosed tarp shelter is suitable for winter conditions is simply NOT far out, unusual, dangerous, or uninformed. Rather, it's pretty common.

Yeah, a winter mountaineering tent with a fly is warmer (assuming you're not going delux with a woodstove). If snow is wind-packed to the point of solid ice (plausible only in exposed places well above treeline in winter) then yeah, battening down the edges of a Megamid (or similar) with chopped ice would be a trick. Otherwise, snow banking seals them right up.