My $0.02. I think the issues around climbing and trail hiking are not the same but since I don't climb I might be wrong. On an established trail, like the MWMT, if there are no issues about one member of the group feeling sick or lame, AND there are no issues about one member being recognized as the leader (for other than permit regulations), I don't see the problem about splitting up with a clear rendezvous point.
If I take a newbie hiking, which I did recently, I wouldn't let them out of my sight. I would walk behind so I could watch for signs of fatigue. But that's a special circumstance where I am the recognized leader based on experience. In this case, I think we're talking about friends who are equals hiking together.
A few years ago I linked up with someone to hike the MWMT. He had a permit and an extra slot. When we got to Trail Camp he was moving much better than me and we agreed he should go on ahead. Towards the top of the switchbacks I was done and he was long gone up the trail. I described him to someone who was continuing up and asked her to tell him I was turning back, he should not wait on top for me, and I would wait for him by the trail at Trail Camp. It worked out and we hiked down together from TC.
I would not have wanted him to miss his opportunity to summit because I was having a bad day. He could have fallen that day. I could have fallen as I went back down the switchbacks alone. Anyone can fall on any trail on any day. Neither of us "abandoned" the other. I would have been sorry, but not felt responsible, if he had fallen and I hope he would have the same reaction if I did.
