...The under the hood aspect is lost on actual users, they just want it to work.
It's hard to write a software package that pleases EVERYONE. What software do you know of that does that? Even with linux distros - there are several variations to suit the various needs/preferences. Secondly, MS makes the most profit by selling multi-volume licenses to corporations and small companies who tend to buy just more than 1 little license for their single PC at home. Who do you think the focus is on? I would argue that when it comes to the Office package - the focus is on the business/corporate side - NOT the casual home user who uses Word to type letters & such. Again, this is just my guess and not fact of course. That's not to say that they don't give a rip about home users of Office - but when it comes to such a complex software suite - it's hard to please all 200 million users.
This may sound obvious, but I make this point because I think you're looking it from a narrow point of view. When you claim the benefit is lost on "actual users". I think you should reconsider who the end user is -- because it is varied. I used to work as an IT professional in corporate america - and have run across all types of users. Complete novices to power users. I would agree that many of casual users wouldn't really appreciate the full benefits of the open XML format. But this is the same category of users who never did appreciate Office 2003/2002/2000 to begin with! Talk about tapping or scratching only a fraction of full capabilities.... I would show them things and they would say "Man, i never knew Excel could do that!"
Then there's the other side. The power users, the IT professionals, the database developers..&.so on. Professionals who often need to integrate data or import data from Excel (or what have you) with another NON-microsoft application. Those are the "actual users" who have a much better appreciation for the benefits of the open XML format.