From a GA perspective (because we actually enter our own flight plans)
I almost never enter the POB into the IFIS flight plan, preferring to go with the TBN option. The flight plan must be entered 30mins before EOBT... between then and actual departure the number of pax could change (as i've had happen a number of times). It's easier to confirm POB over the airways once loaded up.
We don't all have fangled electronic devices that talk to towers.
If we are going to have a POB-passed-to-towers rule at all, it should only be passed on startup or taxi out. Once that's done, the controller is required to put it in the database, no further action required.
Perhaps pass that one on to the powers that be. If this thread is anything to go by, that suggestion would be a welcome addition to the Airways service.