Microsoft trying to support their legacy products with an open standard is not an oxymoron. It would certainly be a lofty goal and would find my full support.
The problem that they are facing however, is that this goal is not only lofty, but extremely hard to achieve within a reasonable time frame. The old MS Office file formats are not trivial and they support OLE objects. To come up with a truely open format to support this and many other features, some of which haev been created by their 3rd party ISV’s, is very hard.
If you consider now the time and market pressure, Microsoft was in need of choosing between a truely open format and a somewhat documented proprietary format. They chose the later for business reasons (I guess). One issue with a truely open format would have been the problem that public stewartship of the protocol would have further delayed either Office 12 or the implementation of that format in Office 12.
The fact that they are now trying to sell the ‘OpenXML’ format as open is somewhat dubious. Even worse is the proposed ECMA seal-of-approval for a subset of the output of Office 12  and its submission to ISO/ITU-T for consideration as an international standard. ‘Open’ means much more that RAND – see e.g. the Minnesota house draft.
 The OpenXML specification does not include the full specifications for OPC. While straightforward (I am tempted to say ‘copied from Star/OpenOffice’ …), Microsoft could potentially stall, delay and/or deter implementations for OPC through legal means. Office 12 creates OpenXML documents that are contained in OPC files. See here for some more discussions on this.