A painted marker usually with a number to mark the route you are on may be ok but a cairn unless on a summit or at a major path junction and marked on a map is not. They don't tell you what route/path you are on or aid navigation unless you know where you are. One of the reasons they fly rock in to make paths in the peak/Wales/Lakes etc is because by using stone from the location adds hugely to erosion. By making a cairn you are taking rock from where it should be to some where it shouldn't, it also attacts people who don't know where they are to follow blindly a pile of stones that can't possibly tell them where they are, this also causes erosion.
