I can not give you comments about the Glocks, but my P2K has this feature directly from the factory and works very well without disturbing the reliability.
I also use an after market buffer on my CZ75 during practice and range use. I have never experienced any reliability issues with buffers, but when the pistol goes out with me, I put the standard recoil spring without the buffer to ensure reliability.
I can definetely say that they prevent the rigorious slide to frame impact. Despite protecting the major parts, they also protect the small parts such as adjustable sights, free floating pins, screws, springs from small resonance forces distrubuted during slide to frame impact.
Most people do not believe them since most of the manufacturers do not put them regularly on their pistols. But, in my personel experience they are usefull parts with positive effects on the service life of the pistol.
Best.