Because Word files don't work with CMYK, and they have no way to handle logo's in EPS well. So preprint paper is the best.
For digital-only it is OK, but when you hook things to databases, why to generate content with PHP and GD straight to PDF?
Maybe those reasons are why they WOULD need a special version of the logo for use in Word. The only thing they would need is a non-eps version like a high-res tif. CMYK would still print to their desktop printer. Not everything done in Word is a form letter and Word shouldn't be used in a high-volume CMYK printing situation. Most times a Word doc is a one-off like an invoice, fax cover or a report to a small group. Things printed on a desktop printer or to a copier. You can't pre-print a version of your stationery for every layout situation and, like I said, many small businesses would never spend the money to create formal printed stationery in the first place. I don't use Word but I use Apple Numbers for my invoices and I have my logo on them. For only one client, I have to provide a printed version. For all others, I make a pdf and email it to them. They may print it out on their end, I don't know. I use Apple Pages to print any envelopes I might need, which is rare, and they have my logo on them.
I think your assumptions about what people do with Word don't match what happens in real life.