(1) I'm talking from the point of view of a single user, not a sysadmin.
(2) Email clients must be simple and snappy.
(3) I have found they are not very appropriate to keep (and eventually search through) a large archive of old emails.
(3) A database seems to be a good way to keep a larger archive of emails messages (including, of course) attachments. For huge archives containing many accounts there could be efficiency problems, but not for a single user (nowadays I've got about 1GB).
(4) I had found some partial examples about how to parse mbox files to import into mysql, but nothing really elaborated. I'll check DBMail with attention. Many thanks!