I think the reference here is to the copper/plastic fuses that are currently being flogged on EBay, as opposed to the aluminum/plastic or aluminum/ceramic ones commonly found in parts stores (and apparantly OEM). The gist of this is that they claim the brass contacts in the fusebox react with the aluminum and corrode, whereas the brass and copper won't. I'm no metalurgist, but Mercedes engineers certainly didn't do us any favors with the design of these things, as you need fingers under 1/2" diameter to get them in and out, so this claim may be valid.
http://cgi.ebay.com/ebaymotors/ws/eBayISAPI.dll?ViewItem&rd=1,1&item=8011023085&sspagename=STRK%3AMEWA%3AIT