Regarding back-ups:
At which temperatures (cold) do Bic/butane lighters typically fail?
They don't actually fail. They just need to be warmed in your pocket or hand to vapourize some gas. I carry a Bic in temperatures far below freezing and never have been unable to make fire with one.
Why would anyone consider friction to be a back-up to a lighter? Seriously.
It's not about
stuff, it's about
skills. Practicing primitive fire techniques makes you a much better fire lighter regardless of the method at hand -- especially in difficult conditions.