One way or another, in a combat environment, you're going to have to close in with the enemy/hostile force. There's no way around it, no matter how much thinking and planning go into it. At some point, you'll end up almost an arms distance. With the advent of the Fully Automatic capabilities, the ratios will turn ugly. The US Army has come around in doctorine and thought in a wide spectrum battle field like Iraq/Trashcanistan. Accuracy and shot placement, on top of controlled pairs in close combat showed proof of current procedures. I don't see a problem with the weapons themselves, other than premature barrel wear, firing pin issues, and the ever so popular magazine follower issue. Will a young cherry in his first firefight be selecting targets as rounds crack over head? Doubt it. That M4 will snap to AUTO, and before he knows it, a tradition battle load of 210 rounds is gone in under 5 minutes, with nothing to show.
Point being, it's something that is unnecessary to a conventional force. If the bearded boys want automatic fire, let em' have it in their shorty, fire breathing M4's.