I like the approach, and it is certainly reassuring to see that one could always extract one's data with some fairly straightforward XSLT and shell scripts.

For casual movement of data between ordinary users and from app to app, however, a Save As OPML and a File Open (OPML), would, I think, be very helpful.

It might be useful, for example, to get one's Omnifocus data into an Omnioutliner file with the contexts, dates and others details in columns.