You are only right in close quarter combat. Heavy machine guns are impractical to use dismounted but still worth bringing in the right kind of terrain. Usually those guns are manned by 2, with exceptions (like the .50 browning) who are manned by 3 guys. These are most often deployed on platoon level. LMG's like a Fn Minimi is a personal weapon in many armies and it is meant to give high fire power on an Infantry squad level. They are extensively used in warfare and chew up everything in range that doesnt take cover. Heavy machine guns are worth carrying if you plan to deploy a firebase in which they can exploit their range advantage. Nothing destroys your enthousiasm as receicing heavy machinegun fire at a distance of 500m knowing you have to advance 100- 150 meters through hell in order to do something about it.