It is a problem raising during the preparation of upgrading from 0.30 to 0.40, but it may also apply when moving data from one box to another, or reinstalling the box.
There are clear and easy instructions about how to back up mails. Just by following those steps, one can hardly lose any email data.
But MIAB plays more than just a mail server. It is also a DNS server. So if I have 500+ DNS entries it will become an headache when I try to rebuild everything. Also, the email alias list (and the addresses allowed to get the email sent on behalf of) does not have instructions about their import/export if some day you have some reason to rebuild the server (no matter to upgrade/move/reinstall).