Well I believe that filing a FP does indeed constitute "booking out", the only benefit to "booking out" as you have identified is if you are on a local, with no intention to land away. Then yes maybe some kind of "overdue" action could be taken but not if it's A/G?, as I said unless the destination is fully aware of your POB, route & ETA then forget any O/D action being taken as the departure airfield will not be ringing up to ensure you have landed safely.
Filing a FP does constitute booking out.
With respect I don't think it is quite as simple as that. I agree that no formal overdue action can/would be taken.
However let's say a pilot does not return home to his nearest and dearest at the expected time because they've had an engine failure and forced landed enroute and been injured etc. Next of kin would call the airfield to enquire whereabouts. If the pilot had booked out then further enquiries could be made at the destination airfield and if necessary the authorities informed. OK I agree it's not the same as filing a FP but rather than being an archaic requirement or red tape I still think it is common sense. What specifically is the objection to doing it?