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.
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.
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.