Full auto turns your firearm from an accurate point weapon into a "cone of fire" area weapon. There are times when doctrine demands saturation of the "beaten zone," (riots, "human wave," room clearing, etc.) but, as Brainglitch said, it is very wasteful. Consider that even 3-round burst mode turns your 30-rd magazine into a virtual 10-round magazine. You can also destroy a barrel in a hurry.