Snow? Ice? Doesn't affect my shifters... Nore does mud and rain...
I'd be more afraid the brake cables would freeze up or that I would fall on a patch of ice than if my shifters would work under those conditions!
Also where do you get they would not work in snow or ice? You PUSH them with your fingers one direction... The other direction is controled by the spring on the derailleur. If the cable and spring are frozen (more likely to get snow/water and ice into the rear d's spring than into the shifter) then shifting would fail just the same with DT or BE.
Only mechanical seems to be the reason to go but my shifters have 15000 miles on them and a friend is replacing his at 30,000 miles...
Index vs friction... Well if you can't tune your system yourself then perhaps it makes sense. Can't find a shimano d in this day and age? Must be in some third world country trying to find a derailleur or at least someone who speaks english so you can ask for one.
There was already a thread like this...
Conclusion... Choose what you like.