Bayesian filtering that works both ways. In fact we currently take PDF/Word/txt/RTF at my job. I'd actual considering narrowing it down to just Word. The type of person who decides that they don't want to submit in Word is probably a type of person that's not a good cultural fit. And I guess we've already ruled out those that prefer to send their resume via smoke signals only.
As an employer it is more important to find good cultural fits than it is as an employee, since it is typically easier for employees to leave than it is to fire them.
My CV is a PDF because I made it in LaTeX. I did that because .doc is not a standardised format, does not make the promise to look right on any system, and doesn't support some of the nicer typesetting I have going on. In my opinion, forcing people to use a proprietary locked-down format is the kind of thing that should be fought. What really gets my goat is that most of these places still don't accept .docx, even though it is standardised, well-supported, and absolutely TRIVIAL to pull plaintext out of for indexing purposes.
However, because I took the initiative to teach myself an industry standard text markup language in order to make a CV that looks really nice and will always display and print properly (and is distributed in the most widely-agreed upon document format for that purpose), you want to exclude me? I'm not submitting in PDF to be difficult or to take a stance, it's a convenient, standardised and well supported format which I am using for its intended purpose.
Comparing smoke signals to PDF is completely disingenuous, as is bundling together "people who happen to submit their CV to you as a PDF" and "people who would flat-out refuse to submit their CV as a .doc".
I don't want to send my resume as a Word document because I wrote it in LaTeX, and it looks correct as a PDF. I don't understand why you wouldn't do me the courtesy of accepting the PDF.
This seems like a good cultural filter to me (on both sides).
As an employer it is more important to find good cultural fits than it is as an employee, since it is typically easier for employees to leave than it is to fire them.