This sort of blanket response isn't feasible for some people. I have a family member who is a recruiter, for example, and receives countless resumes in response to employment ads, most of them in PDF format. To expect that she first knows a person before opening a PDF simply isn't practical. I'm sure there are other similar examples where people must be able to trust that they're safe opening PDFs from somewhat unknown sources.
This is a big problem and avoiding PDFs isn't an effective solution for many.