There’s a discussion at Slashdot about e-mail usability. Actually, it is an Ask Slashdot thread, with a sysadmin wanting to know how to force users into responsible e-mail management; which to many means saving messages to a place where the client will not get at them automatically, and throwing messages away that you do not think are important enough to keep.
Both methods are so that your .pst file will remain manageably small, which apparently is nice for sysadmins. I know from experience that Outlook tends to have problems with large .pst files, so apparently this sort of management would be beneficial to users too.
There is a school of thought in interface design that says that whatever users come up with themselves is good, and I think this holds true for e-mail.
For instance, my experience is that users typically subdivide their Inbox into several layers of mailboxes, and transfer messages there accordingly. In other words, they manage their Inbox. Also, users use the search function of their e-mail clients. These are good things that do not transfer readily to other methods of managing e-mail. What’s more, throwing messages away, or stashing them in “archives” that the client will not access when the user is searching or browsing e-mail, breaks the user experience of being able to access everything they ever wrote or read using that particular persona.
Of course, from a system administrator’s point of view, e-mail clients could be much and much better. Backing up and restoring e-mail is currently far too error prone. But there is no reason to punish users for using a perfectly usable tool.