"Assuming they've spent $100, $200, $400 on the right Microsoft Office software." That's usually a valid assumption. Add to the high fraction of Microsoft Office users the OO.org and LibreOffice users and it's safe to say almost anyone can open a .doc file.
On pre-Office 97 formats I stand corrected. I knew that StarOffice goes way back but I did not know that OO.org can open antique Microsoft documents.
I believe your comments about margins, but none of my clients have ever had such a problem.
I understand standards quite well, formal and de-facto. Once upon a time there were formal POSIX and OSI standards that many IT departments were forced to purchase support for. Those purchase requirements were met, but the standards were rarely used. POSIX faded into oblivion before ever being taken seriously and OSI was superseded by IP. ODF and OpenXML are both formal standards, and I found it ironic that a prior version of Microsoft Office once had better support for ODF formats than OpenXML. Although more complex than any sane person would want to read, OpenXML is just as much a standard as ODF.
Again, standards are what get used. Once upon a time IE6 interpreted HTML and CSS far removed from standards, but web site designers used the IE6 dialect because that's what was used. As Firefox became an embarrassment to Microsoft, newer versions of IE were forced to be more standards-compliant, not by any standards body but by users.
As for PDF, older Adobe PDF readers cannot open some documents created by newer PDF writers. My clients exchange far more .doc(x) files than .pdf, yet they've had more trouble with .pdf files than .doc(x).
Sophisticated users and corporate IT departments can keep reasonably current software that opens any .doc(x), .odt, or .pdf on any desktop. Less sophisticated users, small businesses, and under-funded IT departments should not be ignored.
Some users need Outlook, Publisher, and/or Access, which makes Microsoft Office obvious. Realistically, the cost of Office is a very small addition to a business PC. On the other hand I just installed LibreOffice on a Windows PC that runs an inexpensive Windows-only application that helps run a business. She's thrilled that she can open .doc files for no extra cost. I don't know what the future is for OO.org with Apache, but both Microsoft Office and LibreOffice are good products, and both have their markets.
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