I just replaced my front rotors and pads with a PowerStop pad & rotor set (bought a set of all four for ~ $150 from RockAuto.com). So far, I am very pleased with the result. The new set-up is smooth and quiet unlike my old Brembo blanks that exhibited extreme judder, especially at higher speeds; the runout on my old Brembos was an order of magnitude than the maximum runout specified by the FSM (0.07mm, or 0.028 in).

As I was doing the exchange I noticed that the my car only has a single pad shim rather than the dual set-up with a "shim" and "shim cover", as shown in the FSM. I guess the mechanic that installed my Brembo's (long time ago) decided to drop the shim cover, for whatever reason ....

So, here is my question: Is the shim cover required? Was the absence of the shim cover a major factor contributing to rotor damage (warping, thickness variation) ? Should I bother to get it and re-install?