ORIGIN:If the flight is really fully booked, an agent cannot add anybody to the flight until places open up (e.g. when someone rebooks his flight). Doesn't even work for HONs (~ GS at UA).
Your logic does not fly.


Not fully true.
I have case when somebody was bumped out from flight to allow me as HON to fly.
But maybe it is rare. But on all cases when my rebooking was needed and there was no public availibity to flight I was always told by HON agent that "this is not a problem for you".
Another point is our booking guaranty (actually should work teh same way) -- used it several times, flight fully booked in C, but I got my seat.