Not sure if this link will come through…
http://www.microsoft.com/office/community/en-us/wizard.mspx?&query=word+master+documents&lang=en&cr=US&guid=&sloc=en-us&dg=microsoft.public.word.docmanagement&p=1&tid=fe0fbeb2-66e8-42dd-83fc-a048a2da3715

Some text from it:
“
The master document feature has been broken in *every* version of Word
since the feature was first introduced. In short, using the feature almost
inevitably results in all or part of the document becoming hopelessly
corrupted. Many could benefit from a trustworthy master document feature. My
reading is that the OP needs the feature and wishes for a version of Word in
which it works reliably.

Because of the new file format, I had high hopes for master documents in
Word 2007. Unfortunately, the last time I checked (SP1), the feature was
still broken (i.e., still spontaneously corrupting documents). I have not
ventured into master documents in Word 2007 SP2, but I’ve read nothing that
suggests it was one of the fixes.

For more on why master documents should be avoided by people who value their
documents, see:

http://word.mvps.org/faqs/general/WhyMasterDocsCorrupt.htm

John McGhie also points to an article describing a “safe” way to use master documents. IMO, the rules that need to be followed to avoid corruption are very difficult to enforce when working with a group of authors, and not all that easy to follow when working alone. In any case, master documents need to be made unbreakable and robust before they can be used with reliability and confidence.
“