Well aimed, well placed, controlled fire is the most effective way to neutralize a target. Even automatic fire, except in cases of point man in close terrain, should be aimed, placed bursts or traversed while from a mounted (bi-pod/tri-pod) position through targets of opportunity or on an FPL/FPF taking into account the heat buildup in the barrel/receiver (a runaway gun sucks, and yes, the AR platform if hot enough will run away). It has been shown in battlefield situations that indiscriminate use of automatic fire does not cause more damage to the enemy, but only expends valuable ammunition at a much higher rate. Thus, my statement that most uses of full auto are not warranted...
...fun as hell, yeah, useful or accurate, notsomuch.