I hope you will tolerate a quick comment from an SLF--but one who spent his formative years on the Canadian prairies where the sort of cold temperatures and powder snow that some have mentioned was very common. At sub zero (in Fahrenheit that is) temperatures, snow does indeed blow off far more easily than the heavy wet stuff that I saw when I lived in the UK.
I can half see where the "it'll blow off" posters are coming from.
But...and it's a huge but...the same soft, dry snow is also a great insulator. Any place you have even a weak source of warmth (perhaps the in-wing fuel system heating mentioned in an earlier post) the thickness of the snow can insulate the parts touching the metal from the extremes of the outside temperature and allow it melt or at least soften. Many was the time that the snow on the bonnet or roof of my car could create a layer of ice or at least sticky, adhering snow before it had a chance to completely blow off in normal driving.
Not a problem at all on a car but a huge risk, I'm sure, on an aircraft. Even worse, you can't tell from looking at the snow what's happening at the bottom of the pile and it would only take one mistake from an "expert" to have disastrous consequences. I have to think that the rules are there for a good reason.