I would agree with the navigation theory. Although when the mist is really thick you can barely see your hands in front of you. I somehow doubt the shepherds would risk loosing their sheep into any of the gullies during a really misty day. I would reckon that they would just sit tight, and build another large cairn to pass the time, as you said, they are really well built.
It is not surprising though that hikers cairns are sloppy. They gasp up to the top of the ridge, throw their pack down, chuck a rock on the cairn and take a rest. I have never actually put a stone on a cairn on the top, or though we built a beauty along the cathedral traverse. Hey st
2, scan that image and pop it onto this thread, our "is that a cairn or what" image.
Perhaps someone can ask one of the shepherds the next time they bump into one, a shepherd I mean, not a cairn