A gas piston system has been proven to increase reliability in military combat. However the stock gas system has worked well for me. It isn't necessary for my purposes.
I see this situation as being similar to the Beretta M9 handgun. I carried a Beretta M96 on police patrol, exposed to extreme weather conditions (wind, snow, icing, cold) and it performed flawlessly, a situation not experienced by those familiar with the M9 in military conditions.