So rather than have someone who has been briefed on the operation of the emergency exit the airline would prefer someone who hasn't been briefed
Seems odd that there is a requirement for a person to be briefed on the operation if seated in the row, but the row can remain empty and no one is briefed. It's an important bit of safety equipment and I'd assume that's the reason for a seat occupant to be personally briefed, to ensure someone knows how to get the thing open, rather than the usual cattle who pay no attention to the general briefing or read the card. Airline opening itself to litigation if no one is seated in the row and personally briefed and the worse occurs, people die because they don't know how to open? Mind you, they may make the Darwin award line up for not paying attention to the general brief and card.