Psychologically full auto wrecks havoc with your mind knowing that a gun firing hundreds of rounds increases the probability of you getting hit thus forcing you to into survival mode. and it works against the enemy as well. No one wants to take the chance of getting killed. The advantage of a whole squad having this means the enemy now instead of having one crew with a automatic to take out now has to worry about a whole squad.
In combat you carry so much so you want that supply of ammo to hold you over from the time you leave the wire to the time you get back into it. Logistics is very limited. So you tend to conserve what you have. And even though full auto is more wasteful and can make you combat ineffective. Most of us tended not to use spray and pray. (at least that was my perception) But rather take well aimed shots so you can save the next round for the next enemy that may come up.
"In the beginning of a change, the patriot is a scarce man and brave, hated and scorned. When his cause succeeds, however, the timid join him, for then it costs nothing to be a patriot."