Keith Bierman writes:
Probably betrays my ignorance of the OP's needs, but I'd have thought:
- tracking who joins is more interesting than those leaving
That depends. In many organizational settings monitoring certain channels is part of the job of certain people. And of course there's the "emergency list" use case mentioned by Steven.
- client side filtering (including automated discarding) allows the administrators to save their sanity ...
Some people just don't get it, though, and they're sometimes very competent in their own areas, too. And some filtering is just plain hard. Eg, I have a couple of committee chairs who think it is a good idea to emphasize the importance of college-level list traffic related to their committee work, that comes on lists we're all subscribed to, by forwarding them to everybody in the department -- I'd like to spambucket them based on list serial numbers, but sometimes there is actual content in the forward (especially dangerous is the occasional gloss like "this means you, Prof. Turnbull" -- since these messages are written in Japanese, that's actually more helpful than annoying :-).