Yeah, it makes everything cleaner because it's dissolving the surface layer off it. ;-)
Maybe it's just that I've got pretty deionised water on tap here (18.2 meg-ohm), so that's particularly agressive at stripping metal. - I quite agree that tap water is probably going to be too full of shite though (especially where it's quite hard). Distilled is probably the way forward.
It's probably best not to think about it, and just have faith that the corrosion inhibitors are doing their job; from a chemical perspective it doesn't need to be metal in the water, dissolving enough anti-freeze will have the same effect... although then you get the issue of too much glycol destroying the plastic parts in the system. - Plastic header tanks are the classic example; if you have too much glycol they go brittle and break letting all your coolant out. Maybe that's what the other type of antifreeze is for now that I think about it. - Organic for metal radiators and inorganic for plastic ones?