I don't even think it's intentional, they had to come up with a file format which supports all the weird historical artefacts in the various Office tools. They didn't have the luxury to first come up with a clean file format and then write the tools around it.
And I bet they didn't switch to XML because it was superior to their old file formats, but simply because of the unbelievable XML hype that existed for a short time in the late 1990s and early 2000s.
And I bet they didn't switch to XML because it was superior to their old file formats, but simply because of the unbelievable XML hype that existed for a short time in the late 1990s and early 2000s.