Spudskier,
When I worked at a VFR tower that closed at night and someone called close to closing time we would acknowledge them, tell them of any other known traffic, and told them that we were preparing to close in X minutes. It is not that the controller doesn't want to stay open until you land, they legally are not allowed to.
The FAA frowns heavily on controllers that keep the facility open beyond the advertised time, I've seen a few controllers get chewed on for doing it. Unless there is an emergency the controller is not allowed to clear you to land after the tower is legally closed. Imagine yourself as a controller having to explain to the NTSB and or an attorney in court if something happened, why you cleared so and so to land after the control tower was legally supposed to be closed.
Mike
NATCA FWA