Yes, full auto is necessary.
Does the line grunt get the necessary training for a full auto rifle? Heck No. Any full auto rifle handled properly is more or less a burst fire weapon. From experience (USMC Infantry 0341/ Iraq 2007) the military does not have the grunts shoot enough in training. Don’t get me wrong, form a marksmanship standpoint, they’re on the ball, but in a combative shooting situation, we're lacking. Think of it this way; It takes 1000 perfect movements before an action becomes muscle memory. It takes far more than that for muscle memory to preform perfictly under severe stress (Like having Akmed aiming a PKM at you and then seeing that insidious grin that says "I GOT A BIGGER GUN!"). Can you imagine the amount of ammunition you'd have to purchase to get that kind of training down? Spec ops guys are known for burning out barrels of their rifles in TRAINING! I have never, nor have I ever met another Marine (Bar Force Recon and Seals) who has ever burnt out a barrel. This says a lot considering it only takes about 5000-10000 rounds to burn out a barrel. Proper training can cause you to go through 50,000 rounds easily.
This leads to burst fire weapons. The M16/M4 series weapon is burst fire for two simple reasons: 1. Not enough training hence the military is going to control the grunts trigger finger for him. 2. Better ballistic wounds. All three of those bullets in a burst are going to land in a palm sized area on the target, even at moderate distances. This leads to better ballistic wounds. While if you shoot me in the leg with a single 5.56, I may be able to walk away, a burst of them in a relatively close area is going to destroy my leg. the same thing goes with internal organs.