For you Beretta gurus - I had someone bring in their brand new 92FS A1 and I noticed that it has a blue insert in the frame below the rails. It looks like it is a sleeve that centers the head of the guide rod.
I have honestly never heard of one needing to be replaced.
From what I understand it was initially designed for the .40 berettas, as it greatly increased their life to that of the standard 25,000+ round count of the 9mm slide. Now if it does increase the slide life on the 9mm model I am not sure.
I have a plastic buffer on an HK USPC in 40. mine is set up a little different though. The plastic bushing on mine goes around the recoil spring. From what I have read on my pistol the bushing is suppose to last 20,000 rounds.