Agreed on both of the above accounts - the pointless ones put there by wooks, knock 'em over. The ones that are necessary for navigation - leave 'em up.
When I was in the Flat Tops a few weeks ago we saw several cairns in some huge (1-2 mile wide) fields that looked very old. They were out several hundred yards from a trail and more or less in a straight line parallel to the trail. Built like 4-sided log cabins with flat stones on top, 3-4' tall. Some looked dug out under the base as if the whole thing was meant for lighting a fire under it so it would be a lantern. There's been sheepherding in that region going back to Spanish times - and there are still herds of sheep run up there with Peruvian shepherds. I wonder if they had something to do with that work. We left them though. Any idea what they were?