I think that it just makes it super easy to condemn anyone you dislike. Anyone can be matched to one or more of the behavioral indicators for NPD, individually, because many of them are just regular human behaviors. Selfishness, callousness, anger, egotism, not taking criticism well, etc, are all normal things to experience, and everyone does them, so if you want to condemn someone you can just subjectively and selectively label instances of that behavior as being an indicator of NPD, ignoring any absent or incongruous patterns or criteria.