Others have touched on the "speed hiker" mentality - then there is the other kind, the once-in-a-lifetimer who must do it now because there will be no other opportunity, and has only x days to do it in...

I have mentioned (I think) before that people who half kill themselves to get to Half Dome bemuse me. Prolly 95% of the folks who do it are not hikers, just tourists who spent the previous few days of vacation wandering around the valley and riding shuttles up to see the sequoias. They will hike a 16 mile round trip with difficult (even for a hiker) elevation gains over rough (livestock pounded, broken asphalt) trail, in whatever shoes they are wearing. They aren't backpackers or day hikers, just people in dress shoes and slacks, or skirts and sandals... I've seen monks do it, and little mennonite ladies in dresses with Nikes under their skirts. They could easily do the 2 miles to Sentinel Dome and be within spitting distance of the same elevation as Half Dome. They could do the 8 mile round trip with less elevation change to North Dome and get marvelous views. The relatively level hike to Dewey Point or any other point along either rim would be easier. But it's something on everyone's life list, apparently. So it is with the JMT or the PCT or any other generally known landmark trail - they just want to do it, not to hike or enjoy, just say they did it.

Someone asked me if I would do the JMT with them in a week. Why? They have a week off. I rolled my eyes and said no, if you go with me, we do eight mile days and take pictures. I'll catch you a fish for dinner. I've been doing it in sections because I don't want to shortchange myself on the experience. Also, I looked at him, and knew I would outhike him - while I'm reasonably certain I could do a 20 mile day (not happily, not without some soreness, but I could plod that long), I could tell he wouldn't. I don't like hiking with suffering people.