In the main series, SMN has typically been a hybrid role, most often having healing abilities as well. (Garnet/Dagger, Eiko, Yuna, etc.) With the exception of FF Tactics and a few other places, summon skills were not usually the only abilities that a SMN had. Because of this, I think most people outside of FF know about summons, but very few know about the summoner as a class, or would be able to recognize a summoner horn.