There's at least one open-source desktop publishing system:
Scribus. There are times when automatic layout is not sufficient, and/or we need some seriously complex nonstandard design changes. Can export to Scribus be considered? Its file format is plaintext XML.
I think it'll be quite useful for office designers to be able to receive/write a bulk of text in convenient text editor in simple to use format and then manually design the appearance of the resulting document.