Your mileage may be different. I didn't work on Word (though I talked to those guys about their format) but I worked on two other apps that used the same strategy in the same era. One, on load you had to fix up runtime data that landed in that part of the data segment. Two, the in memory representation was actually somewhat sparse. This meant that a serializer actually read and wrote less to disk than mapping the file. So documents were smaller and there was actually less i/o and faster loads.

The reason I hated it though was because it was very hard to version. I know the Word team had that problem, especially when the mandate came down for older versions to be able to read newer versions. Hard enough to organize the disk format so old versions can ignore stuff, but now you're putting the same requirements on the in-memory representation. Maybe Word did it better.