An aluminum housing on an alternator is like a heatsink. The heat is disipated from the stator inside into the housing. The amount of aluminum that is cast into the housing has a direct effect on how much heat it can disipate. If you go removing the material via grinding/sanding in preperation for polishing, you reduce the surface area of the housing, thereby reducing the heat transfer from the housing to air. (there's less area of aluminum for the air to touch)
What I said was that a polished housing would have little or no affect on this because there's not that much material lost when you sand the alt in preperation for polishing.
Now if you chrome plate the alternator housing, you just cut off all the aluminum's exposure to air, so now it cannot dissipate the heat like it should. So, it retains more heat causing the internals to overheat and fail. Remember, aluminum is an excellent heat conductor and it dissipates the heat just as well. Chrome plating uses nickel, which by nature will obsorb heat and not dissipate it as fast or efficiently as aluminum.