That depends on what you call "casually pirate". Numbers are important. The massive bulk of piracy occurs with copies stripped of DRM already by those who know how to do it. If some users of a DRMed service will decide to pirate something and will hit a roadblock of DRM not letting them to copy it, they'll quickly find other sources which provide the same thing without DRM. So DRM doesn't prevent them from anything - it actually encourages them to look for pirate sources essentially completely defeating the whole purpose of itself.