Things that occur to me:
1) Yes, they do make a difference - they make it easier to plug in and unplug your speakers.
2) Z-plugs are produced by the several million by the german company multi-contact. £4.99 a pair on various hifi suppliers, a lot less from RS.
3) Silver does tarnish. Not as quickly as copper, but it does.
What I do? I have bare wire (2mm diameter solid core) at the speaker, running directly through both pairs of binding posts (ie the same piece of cable acts as the jumper), done up VERY tight with a socket, to Z-plugs at the amp end (about 0.75m away - monoblock behind each speaker). Works for me and sounds fine, so I don't worry about such things anymore.