Support for seamlessly mixing mbox and maildir, would mean that except for the spool mailbox (INBOX), the user can have one tree of mailboxes, say in his Mail directory, subdirectories of which would be maildirs, and files would be mboxes.
Each maildir could contain other maildirs as sub-folders, and maildirs and mboxes could coexist in the same namespace. The user would not have to define anything special like namespaces, but dovecot would simply automatically detect - if it's a regular file it's an mbox, if it's a directory it is either a Maildir if there are cur,new,tmp subdirectories or just a sub-hierarchy (in which case it will contain mboxes or other Maildirs).
I would like this feature present, as this would enable dovecot to integrate with a common usage pattern here - people use, e.g. mutt, which hides the difference between mbox and maildir from you. You could at any time convert a mailbox from mbox to Maildir or vice versa for technical (compatibility/efficiency) reasons without affecting its usage. -- TA (Tom Alsberg)