i worked at a boat shop and i was a deckhand on a ship for a short time. if there isn't a buffer between aluminum and steel(grease, silicone, or spoke prep) the corosion of the aluminum will bond to the steel and fuse the metals. this is much worse of a problem in a marine environment like where i live. this is why you should grease your seatpost and your quill stems. i've heard of factory build wheelsets that don't use spoke prep fusing after a year or two of riding in rain.

brass doesn't corrode in the same way and will stand up better for a longer period of time. i've also heard they are less prone to threads stripping.

in addition they're cheaper and i don't really need to save weight.