Alistair,
On very cold winter backpacking trips the problem with boots is they can freeze overnight and be really cold to almost useless the next morning. Double boots eliminate the problem as the plastic shell stays out with no worries and the inner liner goes in your sleeping bag. Some people will just put their leather boots in a plastic bag, in their sleeping bag, so they stay wet...but warm. I don't like the idea of something very wet and dirty inside my sleeping bag.
After having a outragiously horrible morning last winter at tucks, wearing frozen, insulated leather boots (it was hovering around 0 degrees) i got plastics. haven't used them much for backpacking yet, but am looking forward to not feeling like my toes and heel are going to fall off!!
although. if its not that cold, your boots should be ok, i have survived many years of using regular insultated boots on overnighters. but....if the temp is expected to drop way down........mornings can be uncomfortable or dangerious.
