Data longevity: this is an important point, which is often overlooked because it’s really only an issue in the (distant?) future. Microsoft has made it clear that it wants proprietary document formats, and inconsistent ones at that. This may work as long as Microsoft is around and developing software that supports files created by outdated products. Personally, I’m more comfortable with my OpenOffice.org documents in XML format because I know that in the worst case scenario, I can unzip the document structure and easily extract text from the XML components. This is technical, but what it comes down to is: my data is easily accessible in the future. It is also easy for third party developers to write tools for OpenOffice documents. Data interchange: this builds on the previous point. MS uses proprietary document formats and seems unwilling to allow seamless data flow between different software from independent vendors. It’s just not in their best interest. OpenOffice.org uses data formats designed to be easily interchanged (OASIS specification), and other projects are cooperating with the vision of open document interchange – e.g. Abiword, and KOffice.
Now, given the rapid worldwide growth and popularity of open source software, including OpenOffice.org, do you really think you’re better off locking your documents into an inflexible, non-interchangeable format (MS Word version X)? I would argue that for anyone who values document longevity and interchange, it’s in their best interest to use software based on open data formats.