When I was in the infantry in Europe full auto was pretty much a complete waste of ammo except for very specific circumstances.
Those were essentially house-to-house fighting and clearing trenches/bunkers. Other than that, there was little use for full auto. For the majority of ambushes, you are going to be at a good range, and full auto is a waste of ammo. In any event, your squad belt-fed MG will provide whatever full-auto you need for ambushes.
I guess if your position is "over-run" then that would be another application, but we didn't spend a lot of time practicing that.
This is all in the context of the G-3 rifle, though (which I only used for part of my military service). I have less experience with an M-16/c-7 on full auto, but for a .308 "battle rifle" full auto really is pretty pointless because it is very difficult to shoot accurately.