I think you're correct. The first time I encountered (and recognized) an em-dash in someone's writing was in middle school, and the person that wrote it was someone that I considered to be academically superior to myself. I noticed though, that a lot of people in the same "smart kids" group would use them; almost as if they had worked together on their papers. Maybe they were just reading different material, but it definitely came across as: this will make my writing "look smart".