David Wheeler has an important essay detailing the reasons why it is important not to let your data (or your citizens', if 'you' are a government department) get locked into proprietary formats, even Microsoft 'open' proprietary formats. Read this and find out why the State of Massachusetts, Toronto's own Scarboro College, and many others have already committed to using software that uses OASIS Open Document.
BTW, I am proud to have been advocating open documents based on SGML and later XML for over ten years.