I'm not Gene Stoner, but I seriously doubt that accuracy was the reason why he put in the direct gas system. My bet is, again, weight. One major purpose of several ArmaLITE rifles was to reduce the weight. That's why the original AR15/M16A1 had components made of plastic and aluminum. This was a major innovation in the day, and the direct gas system allows you to shave some more weight - to the point that the original AR-15 was somewhere around 6 lbs., as I recall.
As for accuracy, there's much speculation as to why the AR-system rifles are so accurate, but as far as I can see not proof. Some say the upper/lower system, some say it's the bolt (I would definintely say that the QC standards of the bolt have something to do with it), some say the gas system which minimizes pressures on the barrel.
Tell you what, take 60 high quality barrels (say, Douglass premium air gauged), put 30 on a normal AR-15 and another 30 on a proven gas-piston AR. Shoot 10 10-round groups at 300yd. in a wind tunnel on 2-3 nonconsecutive days. Then, tell me if the piston uppers are/aren't more demonstrably more accurate than the gas-impingement uppers.
Short of that, AFAIC, the jury is still out.