for me one of clear moments when you need this kind of user it will be in this scenarios:
I have one department which need to manage large files with an external party
(for example marketing department and third party printing facility to print quite large promo sheets - A0)
for this I need to make an user for external party with read/write access to ftp
in "standard" approach you create the user (ftpexternuser_n) and that user is "known by both party's (marketing department and external people) and they use it as FTP user only
in my approach I put all mails which go to this user to be forwarded to my address - in this way I can "pick" any reply or missuses of that resource
it will be nice to have one "role playing" menu to select if the user has access to FTP, LDAP, SAMBA, Ibay's..
something like "user panel access"
another usage for this kind of "limited user" is
I want to have one email address for one department IMAP not group - that department is spread in several buildings and need to receive emails and each one take control only on some of this or first available to serve the oldest mail
for example you have one warehouse with more buildings (1,2...n) and to be easy for customers - before they want to came to pick up goods - they send one email to warehose@yourdomain.net and say that he will arrive at 14 00 and he has to take his goods
each section of the ware house can do his part of the job
or in another situation you have one email as problems@yourdomain.net
to this email you will link several departments - and each one respond to problems linked with department activity.
this are probably several scenarios when you'll need "reduced/limited" users on your server