Rob,
 
your analysis is exactly right.  This is probably the major problem
with the use of LISTSERV.  One could argue that sites should include
a global delete for a user when clearing his account, but that would
generate lots of extra traffic.  The best solution, in my opinion,
is to move the service closer to the user, so that each user's
subscription to mail distribution services is handled by a single
(relatively) local server, managed by the same administration
which is responsible for his account.
 
This is how the NEWS model works.  LISTSERV could work in the same
way, if each LISTSERV carried all lists of interest to users in its
"catchment area".  This could be done by setting up lots more peers
for existing lists.  The picture is clouded by considerations of
privacy and security for list which are not public.  A comprehensive
solution may require additional function in LISTSERV.
 
Niall