This was my original comment:
If a person is late to shul and someone took his (paid/kove�a makom) seat, he can�t make that person leave. If davening is underway and a guest needing a seat took an unoccupied seat, he isn�t responsible to know or have to move for a latecomer that normally sits there. Otherwise no guest could ever sit down without having to worry he�ll be kicked out of the seat (at which point there may be no more available seats.)