Video derivatives are some of the most costly, and in some use cases (e.g. uploading MP4's) don't need to be done at all. That's why there are toggles in the Video SP admin form to turn off each derivative that requires ffmpeg.
However, you can't save the form unless ffmpeg is found and can be executed. So, from a fresh install, you can't turn off the derivatives that you don't want because you don't have the program installed that you don't need.
I think we should only validate the path (thus making ffmpeg optional in the SP) if video derivatives are enabled.