...the rear main...may not have been intended to be a bearing surface, and I have never seen a flywheel face machined smooth enough to be a proper bearing surface. I believe there is designed to be float between shims during operationd due to oil circulation and they are not intended to be stuck togeather during operation...
But they will
be "stuck together" anytime there's any axial load applied, there's nowhere for oil to get between them (or between them and the flywheel) when the stack's compressed, as when you step on the clutch. Moreover, the thrust bearing face is most assuredly intended to be a bearing surface - the reliefs in it are how oil under pressure establishes the only
oil film adequate to isolate the moving parts from the stationary one under load.
I'm aware that it's common to find 3-shim stacks in non-automotive applications, and the explanation you've proposed makes sense as sound engineering - more so in a grease, or non-pressurized oil, environment though.
It may be that influenced the VW engineers' thinking, but note that the Porsche 356 engine which came later, and was derived from the VW design, uses a single shim.