I've built and ran purpose built AR's for hunting, 3 Gun, benchrest 3G (Glassed Gas Gun), and CMP/NRA Highpower competitions. I've ran piston drives in all of those applications. I REALLY haven't seen any significant improvement or advantage for piston drives over DI in AR-15's.
"Self Cleaning" is very subjective. All you're really doing is moving the destination of the heat and fouling from your BCG to your piston system.
Running a cleaner load with an adjustable gas block seems to give me more advantage than a piston drive ever did.
If they didn't add $150-250 in cost, I'd be more likely to buy more piston drives, but it's just not pertinent.