Some lists are set up in such a way that it takes list owner intervention
to add users (Subscription= By Owner or Subscription= Closed).  If a
user goes on vacation for a week, it's better all around if he sets his
subscription to NOMAIL, then MAIL when he returns, rather than go through
the rigamarole of resubscribing.  At least that's <my> opinion.  I'm sure
there are others.  :)
 
Nathan