According to the wags at Microsoft, the reason for doing this is because older versions of Windows are "inherently insecure" and would expose computers running Office 11 to all sorts of nasty evildoers.It seems that if you want to upgrade from older versions of Microsoft Office, but keep your existing operating system, the best option will remain to upgrade to OpenOffice.org which is already available for more platforms than any version of Microsoft Office has been. If you want to check out many FLOSS applications for Windows at the same time, check out TheOpenCD
See also: eWeek: Microsoft Courts a Catastrophe , the Inquirer, kestrelworks.com, YaHoo.com, PCWorld.com Beyond 30.
Related stories: Wired.com