According to Richard Stallman, it's a bad practice to deny the existence of a free software that allows the user to at least play/view or decode their data that is stored using a patent-encumbered file format or that is stored using a file format used by non-free software[1] (I'm not even talking about international standards here, since internationally standardized file formats can't actually prove that it'll be used my free software by default, and as a bad example we have a ISO for Office Open XML[2], which is the DOCX, PPTX, and-so-on file formats).
If free software exists to open the file, then let's use it! However, and also based on Richard Stallman's talk, once you open the file, it's recommended for you to save it using a file format used by free software by default, because otherwise, distributing the same file with a file format used by non-free software will just push society's dependency on non-free software even further. Besides, you can't expect for your "favorite MP3 decoder and player" to always be reliable in situations where, for some reason, people start using a "new version of MP3". Furthermore, most free software projects that provide the capability to open/view/decode file formats used by non-free software are, actually, reverse engineering efforts, and society can't always rely on that, mainly because it'll just solve the problem after it has been created, not before. REFERENCES [1] http://audio-video.gnu.org/video/2013-01-29--rms--valencia.webm [2] https://en.wikipedia.org/wiki/Standardization_of_Office_Open_XML
