I have powerslot rotors that are CAD plated and the plating comes off :-( at the track.
At first they were nice and shiny. After the track even though I get little splotches on the inside of my wheel and I see shiny paint like substance radiating from the edges of the rotor. The rotors are fine but the edges now are not shiny, more like a semi-rusty state.
First them keep them rust free while in storage and so they look pretty when the customer buys them.
Second, they are *supposed* to prevent rust from non-swept surfaces (most importantly the inside vanes) so that air flow is not inhibited by rust buildup. Also, just as important, is so that the rotor had does not rust against the hub assembly (happened to my brother's legend rotors once)
However, most poeple just do it so that when you wash the car the edges of the rotor don't look like crap.
Yup, as Jeff commented, the CAD plating is just there for aesthetic reasons. As for the rust build-up on the inside of the vanes...not really a big deal here, mainly for external aesthetics - no performance benefit.