Seriously - I think that on the Underground they have to staff the barriers if they are in use. This is to deal with tickets that don't work, let through people with luggage and to open the barriers in an emergency. One person per set of barriers is sufficient.
The problem at Forest Hill is that we already have (or soon will have) two sets of barriers, one off each platform and now we have a suggestion of a further set onto the car park. That's three people just to look after the barriers. Prior to the barriers there would often have been no one checking tickets at all, and maybe no one selling tickets.
I've noticed that at remote Underground stations at quiet times, the barriers are left open and unstaffed, unpersoned, unmanned and even unwomaned. Whether there is someone lurking in some corner of the station I don't know. All I do know is that TfL stated that our stations would be staffed whenever the trains were stopping once they took over - which isn't the case at the moment.