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Paul -- just trying to offer a bit of assistance with visuals. These are the bits of "tape" to which you refer I gather. Granted, this plethora comes from multiple models, but this should clear things up. This pvc tape-like layer offers protection from the surface components on the GPU and CPU (note the bumps and small soldered on bits on both the GPU and CPU). It helps prevent the heat sink from damaging those bits (metal to components) while allowing the chips' "face" to touch or come within close proximity (with thermal grease) of the heat sink without damage. This "tape" layer also helps regulate the even pressure applied across the chips and their sensors (this is tested with ASD software from Apple to determine whether or not the heat sink is set in place properly). Without this tape layer, you will likely fail a heatsink/CPU sensor test... and run into problems. You also run the risk of damaging the components on the chips.
TAKE NOTE: the CPU tape has small additional "strips" with adhesive on all four sides... see the pic. Small black pieces down by the ESD kapton tape. Those must be in place. Don't lose them. They usually stick to the "tape," but will occasionally stay on the CPU or fall off, requiring you to place them back into their proper placement. I can send you a diagram of that if you need it. Cheers