OpenOffice.org doesn't really fit well into the .PNG example. For one, much of the value for end users isn't much to do with ODF: it's the ability to interoperate with Microsoft Office.
But even if you look only at ODF, the issue there is more akin to web browsers than a library. ODF doesn't have a specific rendering system (nothing mandates a document be laid out in any particular way to conform to the standard), doesn't have much idea of conformance, and could be implemented in a variety of different ways.
I daresay, though, the main reason there are specific people strongly pro-copyleft is that the community has been burned a number of times now.