I agree. There are some trails that are poorly marked. Other times, spring snow-melt will create what looks like a trail, misleading people. Flash floods can wipe away whole sections of trail. I've had fallen trees, yet to be cleared by trail maintenance, obscure the actual trail and I end up following an animal path or an otherwise deprecated path. Hiking in snow presents yet another problem. There are several places I've hiked above the timberline that are wide-open scree fields with no discernible markings. There are plenty of "unmaintained" trails -- they're on maps, but not the beneficiary of any upkeep. Even with a flashlight, hiking at night in thick cover can be a real challenge at times. Many trails are well marked, but there are many instances in which they're not.

>> If you're backpacking through unmarked wilderness, you need a navigation system, and you know you need it. That's 0% of people to several significant figures.

I semi-regularly backpack through unmarked wilderness with no navigation and no map, and I have friends who do as well. But as you said, perhaps I'm in the 0% to several significant figures.