When Microsoft release a new version of office that has an incompatible file format all of the users have been recruited into a viral marketing campaign. This is especially the case with the new docx format. The receipent may need to upgrade their wordprocessor to read the document. Given that recent versions of office only run on recent operating systems this could force a machine upgrade – a little steep for no real benefit.
This is especially insideous when combined with the default setting of hide known file extensions. Users don’t even know that they are using the new format until someone receives the document and can’t read it. This is a wonderful drain on productivity and the novice user that has just bought their new machine does not understand how to “save as a doc file”.
Now I can understand a file format change if there is a real benefit to the user (say Excel 2007 now allowing more than about 50k Rows) but for a majority of the world this is not of use.