Please note that G140 deals with loose-leafs. G140 also gives instructions for the use of imprint date for editions and facsimile or photocopy editions. There seems to be a booming business reprinting law books. Whereas LC would use the imprint date for these reprints, some law libraries are choosing to use the original date with a work letter to distinguish the reprint from the original. I don't know why some catalogers are bothered that some K class numbers have no cutter numbers and use only dates.
Dates are easy to assign.
The reason for so many dates in the K schedules is that our users like dates. Chronology and date of event mean something to lawyers. They want to know when a law was enacted, a decision decided, a treaty signed. Using the date held for congresses instead of cuttering by main entry tells a user something about the time frame of the subject of the conference. A conference on taxation held in 1980 will not contain current tax code information.