According to this statement by Microsoft, their goal was not to create
a file format that can easily supported by other applications or other
vendors. They mainly focussed on their own advantages. Thus, the
vendor independence that you ask for will not be encouraged by
Microsoft's file format.
It should be pointed out that Brian Jones has never (to my knowledge)
actually given an example of something that MS XML can do but
Sure, other vendors can support Microsoft's file format anyway, but
at a much higher cost and with a lot more technical difficulties.
Some friends and I wrote a semi-technical comparison between
OpenDocument and MS XML.