I guess it depends a bit on how close the bar seats are together. If they're so close that having an empty one between groups is necessary for not being on top of each other, then a bar with only seats A and D empty is at maximum capacity and you'd need to wait for a table. It is a bit like the urinal problem, or like the SEPTA problem (SEPTA trains have a double and a triple seat in each row AB_CDE, and the natural fill order is aecbd).

Peter