There are some airlines where listing is a must:
if you want to fly on BA and you are not listed, they send you to a computer to list yourself and then come back to check-in. It seems that their check-in system doesnt allow to check a passenger if it is not on the passenger list display and it inhibits to create a reservation at their position. Qantas told me exactly the same last december when I flew over Sydney. i had to call to a reservation number to list me and right next to that the agent in front of me put me on the list.
There are other airlines that it is not a requirement, for example Iberia, Air france, Alitalia... On Iberia, the company I work for, there if an option which is to check someone not on the Passenger displays, and any agent can do it.
It happened to me once that I showed up on a Delta flight without listing, and the check-in agent listed me before being able to check me in. I guess their system lets you to create a reservation and at the same time lets you to be checked in only if the reservation is created previously. she asked me my employee number, and when I gave it to her she asked me if I was a delta employee. She told me that if I wasnt, she had to use a generic number to be able to list me (don't know really why she needed my employee number, but I guess that the Delta database extracts the data from the employee number).