A small duck here and there when an official trail is sketchy... Okay. Frankly, even in Yosemite with trails getting faint over rocky areas or in lush zones, if you pay attention, you can find trace of the trail ahead. You just need to stop and look. Blazes on trees, cut logs along trails, flattened grass... there are many signs. Part of the fun of hiking is finding the way, I think. (Know how to read a map.)
Foot+-high stacks of balancing rocks not far apart from each other when the trail is very obvious? That's just saying "Look, I've been here!" It's rock-graffiti, as Bee was saying. Why can't we just leave the scene looking as natural as we can so the next person that comes by can have the same experience. Instead, these rock piles scream "a person was here and wanted to leave a mark!". Leaving a gratuitous rock pile is definitely not in the Leave No Trace ethic.
When we leave an area, I always check to make sure that if someone should come through, they would not know we had ever been there (once the footprints even out with weather - and even then, I try to hike on durable surfaces whenever possible.) That means the obvious: no trash or debris left behind. But also means nothing out of place: if we shifted a rock in a campsite for some reason (which we try not to do), we put it back in its place. I guess some of our desert training comes into play here... There are delicate
cryptogamic soil conditions in the desert. Once disturbed, the organisms can take years to a century to recover! I have never forgotten my lessons from the desert and try to hike in the Sierras with as much care.
Over-turning a single rock, if it has been there for years could disrupt a tiny ecosystem.
Why should we impact our surroundings for no good reason? Why do we go backpacking? For me, it is to get closer to nature, to experience natural beauty. And I want everyone after me to have the ability to have the same experience I had. And I would very much appreciate it if others would be as considerate.