You still have the same gas coming out the barrels gas port. The heat and dirt just accumulate in a different location. You are cleaning a piston and gas block instead of the BCG.
Depends upon the design of the gas block and piston system. I have a Barrett REC7 and it is self cleaning. While that sounds about like a "perpetual motion" claim - I cleaned the piston and gas block after 5,000 rounds and only got a tiny bit of grey fouling on a cleaning cloth when I wiped them both with a gun cleaner.