Yeah. So XML is great, once I read the six thousand page spec. Why is this better than a binary format again?
I don't know that I'd suggest a business keep their data in ODF, either, but it's a hell of a lot better than OOXML as far as having actual migration paths and being reasonable for third-party software to read and manipulate. The last time I actually tried working with this stuff (just extracting stuff from MS Office and converting it to more-reasonable HTML), I tried parsing the OOXML, only to realize that it was suicide without a library, no matter how small the data I needed was.
I expect you're completely correct regarding migration and multiple implementations. I have to disagree regarding typical export/conversion routines.
A few years ago, I wrote an MS Office Open XML to XHTML converter. It ignored some of the original layout as it was just an initial step in converting Word documents to websites. Most of my time was spent on converting complex tables (multi-row headers; row & col spans) to semantically correct and fully accessible XHTML tables (thead, tbody, th, & td elements; id & headers attributes; etc).
The main reference I used was "Office Open XML Part 4 - Markup Language Reference.pdf" which is 5,220 pages. As I was only dealing with Word documents I only needed the material in the first two chapters (1. Part Overview; 2. WordprocessingML Reference Material), and that's 1,629 pages. Even then I didn't need much beyond 2.9 Numbering, so that's down to 816 pages.
So, while the specification totals around 6,000 pages, for many business applications far fewer pages will be applicable.