Kent Newsome cites Ed Bott, who writes about receiving MS-Word documents that inadvertently contain revision history and comments. Granted, I'm not in law, but I would assume that the engineering documents we send to our clients (sealed drawings, letters and reports) are every bit as much contract documents as what the lawyers are sending. But I have to say we very rarely ever send MS-Word documents or AutoCAD .dwg files to our clients. These days, the drawings, letters and reports that go out to our clients via email almost always go in PDF format.
The main advantage to us is formatting control. With drawings, the print quality and line weight issues have traditionally been a problem. We'd have to do a lot of futzing around with pen settings files that would have to go along with the drawings, and depending on the recipient's printer, we'd still have no guarantee that what they printed would accurately resemble what we intended. PDF solves that problem for us. The PDF we generate from AutoCAD (I use CutePDF) prints the same on the recipient's end as it does in our office. In addition, we can protect our PDF's so we can more securely send drawings with a P.Eng seal and signature - although the effectiveness of this is still a hotly debated topic among consultants.
If the documents we're sending are letters or reports to be reviewed and marked up by the client, they either have to do it using a PDF editor, or they hand mark it and send it back. If CAD drawings and plans are to be marked up or used by the client, contractor or another engineering consultant, we will send them AutoCAD .dwg files, but those drawings have the P.Eng seal and signatures removed (in fact those things are linked to documents on our servers and never get sent out anyway). But any drawings containing signatures and a seal are sent out in PDF format or hardcopy.
While we do use .doc format internally for some things [I try to use OpenOffice of course ;) ], I am always surprised and a little amused when someone sends me a Word document via email. Almost everyone in our business seems to have moved to PDF for document exchange.
Maybe Ed and Kent are talking strictly about clients being able to make revision-trackable comments and changes to documents. In that case, I can see the familiarity of MS-Word being an advantage, but the lack of protection, formatting control problems and the inadvertent escape of sensitive metadata makes me wonder why it's still the format of choice for them.