I think 31 is referencing specifically 416 pattern rifles. I'm not completely aware of the field stripping SOP for the 416, but nothing I remember of the rifle requires you to pull out the bolt group to take off the piston.
As 31 describes, a piston system without a piston will act like a standard Direct Impingement AR platform without the bolt cycling (as no gas would strike the bolt-face), causing you to have to use the gun like a bolt-action.>>31
I agree. Piston guns are really good, keeping most of that gas out of the bolt group and decreases carbon fouling. However, because of how less... "violent", the bolt cycle is, I've seen 416's end up not as reliable in extreme-mud (well out of standard "what the gun should be able to do" keep in mind).
Everything has a positive, and a negative. I for on prefer Direct Impingement for its simplicity mechanically speaking, but I wouldn't hesitate to pick up a 416.