This devil would be a human being disguised to teach a moral lesson with a theatrical metaphor, so let's not take the metaphor too far. I think we would do well not to say that users are “selling their souls” if they install nonfree software—rather, part of their own freedom is what they forfeit. We don't need to exaggerate to teach the point...
I'm sympathetic to the FSF's goals, but I think they're desire for ideological purity prevents them from achieving meaningful wins. Increasing the availability and desirability of Free software shouldn't have to mean total exclusion of all non-Free software.As an industry, I think we should be more troubled by devices that aren't even allowed to run code without special permission from some third party. The App Store is a sad state of affairs in my opinion.