KNS pins are not needed when you have high quality lower receivers and high quality triggers... but truth is, lower receiver quality varies wildly.
At a class, I was next to a guy whose hammer pin walked and he had uncontrollable full auto (as the hammer was just out of alignment enough to miss the disconnector). His lower receiver from one of those "Billet receivers of the month" companies was out of spec. I've also heard of it happening with forged lowers too. For them... the KNS is a must have, for most people, it's piece of mind.
A CORRECTLY anodized, HARDENED, 7075-T6 lower receiver should NEVER have its fire control group holes get "worn" or "rounded out. That is why you have 1967 Colt AR-15s running strong... and some of the first MK12 SPRS issued in the GWOT built on M-16A1 lowers from the 1960s. Thing is... even today, in 2015, anodizing quality is across the board. A lot of supposedely "milspec" anodized lower receivers are not mil spec anodized at all, and they can eventually wear out.
For my personal lower receivers, I prefer to buy from companies I can trust. Companies that do anodizing right. Colt, LMT, Knights.
I had to help a local PD destroy their Colt Commando lowers once (leadership's call) and the band saw took forever to cut through the decades old receivers. Aluminum Oxide is 9/10 on the Mohs hardness scale. In comparison, I cut a billet receiver from one of those random billet companies in the name of science and curiosity and the band saw went right through.
Buyer beware.