With the processing power, RAM and storage space of today's computers, the reason for Master Documents (and demand from corporate users) is minimal. They tacked the feature on in the 1990's to compete with Word Perfect (which did such things well) without really finishing it. It remains unfinished, and, as Graham said, defective.

Master Documents "Feature"

"The jury is in: The Master Document feature in Word 2010 remains word processing's answer to Conan the Destroyer. Use it only if you enjoy pain and frustration." Microsoft Word 2010 Bible by Herb Tyson, MVP.