Cairns are small stacks of rocks used to mark trails. These can be especially useful above the tree line where other trail blazes would be inappropriate or unsightly.
This answer discourages their use in some circumstances as their use creates paths where there previously were none.
In which situations/circumstances/settings is making a cairn, or multiple cairns, appropriate?
On designated trails that are infrequently traveled and maintained, using them to mark a faint or overgrown section of trail, the point a trail passes under a large downed tree, a switchback, a trail junction, or a creek crossing, where the tread of the trail itself is not visually clear ahead, is generally acceptable and helps keep people on the actual trail.
What are the issues?
There are two issues here: the siting of official navigation cairns by whoever is responsible for maintaining the trail, and the building of unofficial cairns by visitors.
Official cairns
For official cairns, attitudes to waymarking vary widely in different countries based on local traditions and conditions. The locals will build and maintain any cairns that are required, and their decisions should be respected.
In the UK we are broadly influenced by "Unna's Rules", which emphasise that land should be left in its natural state so far as possible and that walkers should rely on their navigation skills. Even official cairns are kept to a minimum.
Unofficial cairns
For unofficial cairns, it's not the role of visitors to alter the landscape. In the context of Europe, or of highly trafficked trails anywhere, I can't think of any valid reason for a visitor to build a cairn.
These unofficial cairns are becoming a genuine menace here in the UK. In heavily used areas like Ben Nevis and the Lakes, local rangers and volunteers have to remove literally hundreds of cairns a year. They damage the landscape and can be misleading if not well sited. The new fad for stone-balancing isn't helping either - it's important that people dismantle their creations after they've been photographed.
Please leave no trace!
So to summarise, if you're part of a ranger service or club maintaining a trail, it's more than likely that you will be following well established local guidelines honed through generations of experience. If you are just visiting, please leave well alone and enjoy the landscape as it is.
I would generally agree with the "as few as possible approach", but would add that if it is a question of safety or avoiding lots of time wasted (which in turn may morph into a safety issue), then I think markers can be helpful.
But I don't think it should be exaggerated - markers can be used instead of cairns.
For example in Vistasvaggi (Vistas Valley) in North Sweden, there are fairly inconspicuous markers in the form of two half buried brick-sized red-brown rocks close to each other. I didn't understand they were markers until I had seen a few.
Of course you can argue it is hard to get lost even in a wide valley and it certainly is at large scale, but I found it helpful to get the detail right when going over soggy ground, tired and damp.
