I've always wondered about this... I mean, Eugene Stoner is a genius weapons designer, but this just seems like such a bad idea from the very begining! Yes, the direct-gas impingement design will have lower reciprocating mass, with lighter weight, but it shouldn't be enough to really matter. The design allows combustion gasses to travel all the way back to the locking lugs, which are also needlessly (IMHO) complex! And bad things will happen to the shooter if he or she is left handed and have a case blowout! Plus, the idea of such a long, thin gas tube that can foul just seems dumb. And then there's the need for a "forward assist," which I've read that Stoner opposed.
It just seems that so many problems could have been avoided with the use of an operating rod (and maybe an operating rod handle too). Ruger decided to use it on their new rifle.
I happen to own a AR-10 without the forward assist. Much of my shooting is done in very very gritty desert conditions on a large dry lake. The fine grit there gets into everything and is a pain. One failure I've had several times if a failure of the bolt to go into battery. Because of the design of the AR the gun can not be disassembled when it fails in this way. The bolt if short of full battery by as little as 1/3 of an inch or so prevents the upper and lower from swinging apart when the takedown pin is pushed free.
To disassemble it I have to take a small allen key, put it into one of the holes in side of the bolt that is visible through the ejection port and pound the bolt forward.
The forward assist is a much faster and simpler way to accomplish this. I think the Army did well to insist on it and I would be reluctant to buy another AR pattern weapon that lacked it.