You are mostly right...
The idea behind <i>non-conductive</i> shims is that of keeping the heatsink stable on top the CPU.
Here's an example of shim-done-right... <A HREF="http://www.casecooler.com/unnonmicshim.html" target="_new">http://www.casecooler.com/unnonmicshim.html</A>
The problem is pretty simple... the spring clip mounting method relies upon pressure between the flat bottom of the heatsink and the flat top of the CPU to keep things in place. This works very well <b>if</b> there is enough pressure, the heatsink is flat, the cpu is flat and the system is not moved or is moved very gently.
However:
AMD's silly rubber feet do next to nothing to stabilize the assembly.
Of 19 AMD XP series cpus I have in stock here, none of the heat islands are flat. The heat island on every one of them is slightly domed.
Of the half dozen heatsinks I have here, only 3 are truly flat bottomed (the others will be before I sell them
)
There is no amount of single point pressure that will stabilize an object on a curved surface and in fact pressure tends to de-stabilize them. Given that spring clips create a pressure point about the size of a pin head and the sides of the spring clips are uneven in length, the expansion due to heat is also unequal, so the heatsink will actually move on top of the CPU as it warms up... which can repeatedly break the thermal bond between heatsink and cpu.
Of course ALL computers get moved from time to time... dusting, rearranging, bumps and drops, etc. With nothing more than those silly rubber feet to keep things from tipping, rocking or bouncing, it is a pretty simple matter to disturb the heatsink even in drops as small as 1/4"... like when you are getting your fingers out from under the case.
The non-conductive shim serves no real cooling purpose. It's job is to eliminate this mechanical instability inherrent in the mounting method. With a correctly made shim (.75mm thick, contacting the edges of the chip) when the heat sink is put on an AMD processor the chip will flex just a bit and some, not all, of the pressure of the clip is transferred onto the shim, creating the far more stable mounting situation of having a single point of pressure applied in the center of a large outer rim.
By way of a somewhat extreme analogy... try putting a ruler on top of a pingpong ball and then holding it in place with the point of a pencil, note what happens when you increase the pressure... Now put the ruler on top of a glass and try holding it in place with a pencil with varying pressures... see the difference?
Conductive shims are nothing but a really good way to short out your CPU.
--->It ain't better if it don't work<---