I know NWA use to hold two F seats on almost all flights for FAMs. It made life easier all the way around. Since they often book seats at the very last minute, so it is easier to have the seats ready and then put them up for UPG versus to have F booked and then need to figure out where the two seats are coming from. Also it helped a lot for the top elites' buying last minute tickets/ or Irrops since there were at least two seats to UPG.