Why direct gas impingement?
The discussion or argument about DGI vs. piston actions is a field of landmines. The main problem is to dispell misinformation on both sides. Perhaps this article best expresses NGA's viewpoint.
In addition to Rob's points, we believe pistons raise a special problem for accuracy in precision shooting due to the effect of mechanical components moving on top of the barrel at the time the bullet exits the barrel. This can be a thorny technical discussion as many people assume the DGI system has no compressive force on top of the barrel but that's not true. It is likely that a piston system can be designed that has no more effect on movement of the barrel at the moment of bullet exit than does the DGI system. Perhaps there are one or two designs already out there. It's really impossible for us to know for sure based on what's being published at this point.
Meanwhile, we stick with what is tried and true and improve on it.