747-436 - As an area controller I can only summise what happens on the ground, and whilst it is true that landing rates drop this is more often than not caused by pilot limitations. ie the plane is certified to land in nil visibility, but the pilot might not be.
Don't forget 'though, that weather can cause us radar controllers a huge headache, and very often departure restrictions are imposed, as well as arrival restrictions to protect the area (and approch) radar controllers whose workload increases dramatically. Therefore restrictions are imposed to limit the number of a/c in the airspace so that pilots requests for weather avoidance can be more easily accomodated.
Hope that helps! The human factor thing is a valid one, in answer to your question. The answer would perhaps be an automated air traffic service which would then remove the human factor! This (as I am sure ATCOs who are reading this are shouting at their screens)would be nigh on impossible. Imagine a computer trying to deal with unlimited numbers of requests for heading changes due weather, via an ACARs link (or whatever gadget would be used to contact pilots) Not realistic!