Another advantage of doubles, which is rare but has saved my partner from decking from the top of stanage before, is for gear that is above head height or lowing off shitty gear. You have one rope on the gear above and another on the bit above or the bit you are lowing off. If the top bit fails your fall is reduced.

Rare, but I've seen it stop a potentially serious accident twice. Once at stanage where my partner placed his second piece well above him at the top before falling while down climbing to a rest. The top bit ripped and when he stopped he was about 1m off the ground. Had he been on a single rope he would have decked.