Here's a good tip/hack for my quandary about backing up Mail.app email messages into a text-based format outside of the Core Data database, as part of my project of becoming less entangled with Mail.app.
A mostly unknown feature of Mail.app is its ability to export a Mailbox into the everything-reads-it MBOX format. Perhaps it's the glaring lack of an "Export Mailbox" menu option. But in true Apple tradition, just drag your Mailbox to the folder on your drive that you wish to export to. That's it.
(from Koru Productions.)