To Cairn Or Not to Cairn: What's Your Take?
When I first started hiking as a hobby, I didn't know that building cairns was a thing. In fact, I didn't even know what a cairn was. When I started visiting the national parks, it seemed like cairns were everywhere - and the protest against them. Like anything else, there are two sides to the argument:
For:
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Trail markers: In places where the path isn't clear, cairns can guide hikers and keep them on track.
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Nothing new about them: Cairns have been used for ages as landmarks or memorials. They're quite historical.
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Helpful landmarks: In tricky areas like rocky terrains or open fields, cairns can point out the safe way to go.
Against:
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They're invasive and disruptive: Moving rocks around can mess with animal habitats and lead to erosion.
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They're confusing: Too many random cairns can mislead hikers and send them off the main trail.
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Leave no trace: Building cairns just for fun goes against the "Leave No Trace" principles.
OK, so without insulting anyone else's viewpoint, tell me what you think. The smartest answers get extra points. 
This is such a great question! Definitely goes back and forth in the outdoor world.
I think in the cases of true, experienced hikers, it's usually okay. If you're an avid hiker and you're out on a trail that already has some cairns and you come to a place where you lose the trail a bit, that seems like a reasonable time to build a cairn.
If you just think they look cute, and you're trying to practice your rock-balancing skills or create an aesthetically pleasing photo, please don't build a cairn. People really do rely on these as a navigational tool, and the trouble an inappropriately placed cairn could cause should not be underestimated.
Just my opinion!
I agree that they can be helpful for navigation, but should only be placed by experienced hikers. I think there are moments where they can be aesthetically appropriate, for instance, the memorial to Thoreau at Walden Pond. However, overall I think people should refrain from building them both for the environmental impact but also so that hikers don't end up confused and lost. When solo hiking in Arches I relied on Cairns pretty heavily and am lucky there weren't any false ones that led me astray.