Hey Swen:
My best guess, and IMHO:
Copper prongs on plugs would be to "soft", and would be an additional cost factor in mfg. Brass was/is used, also brass plating, and nickle plating on whatever base metal the mfg uses.
Im'm not a metalurgist, but I don't think there is a problem with any of the dissimilar metals you mentioned.
Also, you stated that you have seen "rusted" attachment plugs, I can't recall ever coming across any. I'm not doubting your comment, but are the ones you found a "factory" plug, or an aftermarket plug?; and are they from the USA??
John
