Malibuite: it's for looks. I had Brabus brushed aluminium door pins on my R170, and it does look better then the standard black ones. Also, from a distance, it's easier to see if you locked your car, which is nice (so that you don't need to go looking for your keys if you were wondering if the car was locked or not).
On the R170, I think it was an unscrew/screw back on deal to change doorpins.
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