Under rare circumstances, I've asked an agent to uncheck me in on my last flight of an itinerary when I suddenly felt to need *not* to fly. Maybe I was ill. Maybe I had an emergency in my intermediate city. The important point is that I don't often have an emergency mid-trip. Asking an agent to uncheck me in is the considerate thing to do. Delta can re-use my seat, and the gate agent does not waste time paging me wondering if I'm going to show.