For civilian use, you are correct for about 99% of FA uses. The primary reason to own them is fun and bragging rights. Few BGs carry them and even fewer are skilled in their use. The North Hollywood shootout a few years ago was an example of BGs that had some skill in using their machineguns, yet they failed to kill anyone. The only thing they really accomplished was to keep the LEOs stuck behind cover. They had little chance of escaping. Compare that to the '86 FBI Miami Shootout where two BGs, one with a semi-auto Mini-14, killed 3 FBI agents and wounded 4 more. When the 2 BGs were killed, there had climbed into an FBI car and were trying to drive away. Both BGs had sustained fatal wounds during the gunfight and would have died soon anyway, but they still came close to actually escaping the scene.
There is a reason why the M16 is no longer full auto, 1st two rounds are ok, the rest are up in the sky.
An M16 on full auto is very easy to control and maintain on target. The reason the military went with the 3-round burst is because they didn't want new guys to waste bullets in combat and a mechanical fix on the gun is cheaper than teaching GIs trigger control.