I suspect this configuration's popularity arises from one of many audiophool fables: "skin effect" - if electrons only travel on the surface of a conductor why not benefit from the most conductive metal there is without the expense of making the whole conductor from silver?
Problem is, when two different metals are placed in proximity, minute differences in electrical potential can cause bimetallic electrolysis, with one acting effectively as a sacrificial anode - though not a structural problem in coated wires, it's still usually necessary for this reason and to make an effective and permanent bond when plating a metal to use an intermediate such as nickel. Thus you end up with a sandwich of two good conductors effectively separated by a less good. For this reason, theoretically, silver plated wires are inferior to plain copper, and indeed in cheaper iterations such as the notorious QED silver anniversary this seems to pan-out in some of our experiences.
However, like all good broad, sweeping generalisations, there seem to be exceptions - the second best wanky wire I've ever used - TCI Cobra - uses silver-plated copper conductors, and is a superb interconnect. However it uses a teflon dielectric as opposed to the QED's polyethylene, and causes me to echo countless predecessors in wondering if the quality of the dielectric isn't at least as important, or even more important, than the quality of the conductor (within reason) ?
Only experimenation in your system will give you a personally meaningful answer to this...
churz, eofs