IMO, if the weapon gives controllable FA - like the Ultimax - it would not be pissing away ammo. The problem lies in the fact that light weight rifles firing a high velocity round like the 5.56 are harder to control.Full auto in a rifle is generally just a good way to piss through your ammunition and not hit much - especially with a 7.62mm rifle!
While the first true assault rifle the MP44 was allegedly able to give controllable FA that impressed the Russians, with the M16 we have to go back to semi.
In the book "Combat Battalion" about the Aussie 8RAR in Vietnam, author said that FA was necessary cos in the jungle, semi aimed shots were not effective at hitting fleeting targets. And this is on the rare occasion you actually did get to see who you are shooting at.But why rip off a 4-5 round burst at some one when a controlled pair fired on semi-auto is going to do the job nicely...snip... The selector on my M4 never went past semi the whole time I was in Iraq - there was no need.
Also mentioned was the fact that in the jungle, high volume of fire - especially automatic fire - gives a psychological advantage when neither sides can actually see each other through the foliage.
Also in reaction to an ambush - semi just don't cut it.
I guess Iraq terrain is a whole different ball game?
In our training, we were allowed FA when clearing rooms but otherwise it is a chargeable offence to rock and roll...