People make that same argument for many components of the guitar. I think the amalgamation of everything affects everything in some way, no matter how slight. The brass shim seems like a neat idea. I agree that it may not really make much of a tonal difference at all, but the structure of the material itself may be a bit more sturdy than wood, especially for the extremely thin shims, which break easily under pressure, especially if you have to remove the neck to adjust the truss rod (Like on mine).
