I'm a long-call reserve commuter. One problem with time-of-check-in is that I must list for flights I don't intend on taking in order to have a shot at a seat should I need it. I cancel, but this creates more churn and uncertainty on the list. Line holders have the same problem finishing a trip, especially narrow body guys plagued with delays. With reserves going to work (many senior to line-holders in other categories), and narrow body guys going home, a time-of-check-in priority would make the list so chaotic it would essentially be a meaningless but very time-consuming game.