Yunus - What you are looking at is the gas tube that directs the gas from the barrel back to cycle the action.
Yes, this is a "dirty" operation - but you would have to be slinging thousands of rounds a week through your average AR to get to a point where build up would result in a FTE.
One way to counter act this is to take an average pipe cleaner and run it down the gas tube every once in awhile. Works wonders.
Now, as for the gas piston system, here are the basics....
Yes, the system is cleaner and yes the system is "more reliable" because you now have an operating rod driving the action instead of gas pressure.
The operating rod is encased in a type of gel that keeps it running when super hot and is more along the lines of the AK, where the constant yammering of heavy full auto fire won't cause a problem.
The down side is, anytime you introduce another harmonic to the front end of the weapon, like an operating rod, your accuracy is going to suffer. There is no way around it without throwing thousands of dollars at the problem.
So, while the .416 is an amazing weapon platform, it's not as inheriently accurate as a true, well built, gas driven AR.
Will it still hit Minute of Bad Guy?? All day long, and without issue. Is it worth switching over to a piston driven upper? Sure, if you have about $1500 burning a whole in your pocket and you want a hard core room cleaning gun.
For basic target shooting, putting 500 rounds a month down range, doing some basic cleaning and due dilligence on your weapon - I don't believe it's "needed" by your average shooter.
If I was going to the sandbox, and I knew I was going to be throwing a TON of lead in full auto applications, I would consider taking a piston upper along, but it wouldn't be my primary weapon system.