For those that aren’t aware, this is the origin of Southwest’s quirky boarding process. They required you to check in at the gate, and each gate had a set of reusable plastic boarding cards with sequential numbers— You were handed the next card when you checked on and gave it back as you got on the plane, so that it could be reused for the next flight.
Everyone boarded the plane in the order they arrived for the flight without needing to stand in a strict queue while they waited. If the flight you were planning to take was full, you would be first in line for the next one which was often scheduled for less than half an hour later for their core routes.
That said, what's the point of the buddy pass system, if it's not guaranteed? Who goes to the airport, through security and checkin, thinking "maybe I'll fly today, maybe not, maybe I'll fly somewhere else, whatever's available, you know?"