I didn't know that. I presumed wrongly that Mail.app wouldn't allow me to re-export my stored mail. Now I have the dilemma of choosing to go back to Mail.app (which I really liked) and lose my Thunderbird customization, or stick to Thunderbird :-)
Not really. If google somehow loses your mail, it's not like you'd switch to using mail.app. You'd use it to upload your mail back to google or another provider.
Mail.app quite easily saves messages (or groups of messages) as standard mbox files. Select the messages you wish to export, go to "File -> Save As..." and select "Raw Message Source" as your desired format.
If you're just looking to move messages to another server though, it's almost certainly easier to add a different mail server and drag/drop your messages to the new server.
Even easier if you want the whole thing, just right click on a mailbox and select "Archive mailbox...". It will save a copy in the mbox format in the location you choose.
