The are very, very useful in pistol-caliber ARs, because blowback-operated systems are very hard on hammer pins. I have a set in my AR45 and the 9mm AR. I have a spare set of them in the toolbox, and they'll be going to use pretty soon.
I have a decent stainless steel trigger system that i did a little polishing to - it's not special, but it is a decent trigger. I've got the perfect build going on that I can use it in, but the damn thing broke the j-spring in the hammer. it won't keep a hammer pin in place to save it's life, and as soon as it starts to walk, the rifle doubles - one pull, two shots. Not good. The spare set of KNS pins will be used in that build, with that trigger, for that reason. J-springs aren't easy to find, fast to get, or easy to change. Fuk the J-spring replacement, or throwing a3way the trigger. KNS pins will save it.
You better bet that if I had a registered full-auto lower, I'd have KNS pins in it, and i wouldn't run a blowback-operated AR without them.
They have their place.