A few years ago, I moved from programming to a creative/prose-based career where I interact almost exclusively with people who use computers as mostly-replaceable appliances. In this world, interop with MS Word is a baseline assumption. In my experience, there are many fields that share this characteristic.
If Emacs wants to be usable for people living in this world, it's not necessary that Emacs supports all of the various features of doc, docx, or odt files, but it is necessary that files not be corrupted (to use Richard's term) by passing through Emacs. Several years ago I cobbled together a partial solution that used a set of external (Java-based) converters, but it proved to be fragile and eventually bit-rotted. I believe that system was based on the idea that the XML-based doc variants would end up being more open (not free, but published specs) than they actually were.
From my experience, the next-most important feature for interop is support for "track changes", as its use is standard practice in many fields, and anything that doesn't support it is not workable. When last I looked, the details here were never published by MS, and would need to be reverse-engineered.
I hope this helps,
~Chad