If I understand the "rules" rightly from discussion here, every plane takes on board fuel to go to the alternate. That sounds very good, but if your destination is a big airport that suddenly shuts down for some reason while you are already circling over it, and the alternate is a smaller airport that cannot handle such traffic, exactly where are the guys short on fuel supposed to park their bus?
Does anyone keep track of how many aircraft have Airport-X as an alternate at any given time? With computerization this shouldn't be too hard.
That might give a more robust system.