I actually mentioned this rule a while back....as a holdover from another system.
A particular field can only handle and service so many aircraft.
A small field could handle so many, a medium field this many, and a large field, this many.
That would provide a solution to this problem. However, it could provide issues depending on how you station your pilots, and with fuel being an issue in many FSO setups, it could get really ugly, really quick.