"The main reason people repeatedly "roll" their ankles is not so much a question of weakness, although that does play a bit of a role. More importantly is proprioception - your body's ability to know where it is in space. This means when you are about to put your foot down on the ground your body actually knows what position the foot is in before it touches the ground."
For backpacking, I might have thought that the ability to make corrections smoothly after touch-down would be as important or maybe even more. Other than when I'm on talus, I'm not normally looking closely at where my feet will go, and even if I am, I don't necessarily know what's going to happen when I put my foot down (rocks and holes hidden by grass, who knows what under leaves or snow, etc). Might this be the case?
In keeping with the theme of one of Jen's pearls from another thread, for me, the best training for walking on uneven ground seems to be .... walking on uneven ground.