A guy did a torture test on y-tube and it took some 900 rounds of continues firing and melted the rod off and the gun still keep on firing. On the other hand some people like the way it looks, personal choice. Some earlier gen 4 had a spring problem where the spring keeper broke and wouldn’t let the gun cycle right but they corrected that
Stock RSA is only about 7 bucks I think. Your factory RSA with heavy loads should go about 4500 rounds before you see any issues with a completely dry and dirty gun. The factory part is pretty good. The stainless steel one is only needed if you want to run aftermarket recoil springs such as Wolff. The most common reasons I can think of is someone that is running some very lightly loads reloads they made themselves and needing a lighter recoil spring or very hot reloads and wanting a heavier one.