Which is the crux of the problem. We tend to buy weapons that are 80% of the best because they are 50% of the cost.
Heck, I'd prefer we let our soldiers take their own personal firearms with them to the front (provided they were of one of 2-3 standard calibers). Chances are you'd have better-maintained, higher reliability weapons best-suited for each soldier!
I don’t know that any of the various piston systems are better than a standard AR-15. I would probably buy one if I thought they were noticeably better. One of the things I really like about AR-15s is that they are very accurate. Does hanging a piston off of one have any effect on that? I can see how it could if not done correctly. I personally have not experienced any reliability issues with the present AR-15/M-16 either in the military or with my civilian models aside from some issues caused by using old, beat up military magazines.