Mails were once mostly stored as a file in Mbox format, then in a directory tree in Maildir format. Now Deltachat (and at least one other mail client) store mails in an SQLite database.
Would a third open-standard mail-storage format be a good idea?
The usual benefits: reduce duplicate work, improve interoperability, make archival storage more reliable, etc..
Delta Chat is not storing raw mails, it only stores decrypted and decoded message parts.
Deltachat stores and displays the decypted body, a timestamp, the sender and the recipient: the bits most people care about. It seems reasonable for a standard to allow dropping the rest.
Though an encrypted database would also be a useful option, and for some purposes you really would want every header saved (presumably with compression, as they can be quite repetitive).