[Morrigoon]

As a fellow polar bear my biggest concern is how much they heat up interior spaces when it's cold out. Drives me nuts to get so hot inside. So while longjohns could very well come in handy, being an undergarment it's less easy to peel them off. I'd say at least get the bottom half, since there are only so many layers that can be added to our legs. The top is up to you. Might be good to have just in case, but in general you will probably prefer to have short sleeves under a big jacket or sweatshirt then jacket. So when you get inside, you can regulate your body temp by going down to shirtsleeves (should balance out the longjohns on your legs)

I'm rambling, but you get the picture.