I've got a small supply of precious metal wire left over from my old wire wrapping and beading days. I've used bits of the copper and brass inside sculptures as support and as long as the polymer clay is completely sealing the wire inside it works fine as a bit of armature.
But I've discovered that the clay itself does not bond to the wire in any way and there is not enough shrinkage to hold the wire tight. So although I've used wire to help keep embellishment beads on pieces it's not really holding the two pieces together, even with a spot of liquid sculpey added for extra stability (it doesn't bond to the wire either).
This means that eventually the bead can pull or fall right off the piece. A lot of places recommend using a super glue formula to put it back, but I've found that unreliable too, the only thing it's good for is holding a couple pieces together until you form a better clay bond and bake it.
Wire can still be used, but the ends need to be bent up at a 90 degree angle at the tips before inserting. That leaves a bigger hole but you then pack the clay around the wire and it forms a wall that the bent tip can't pull through.
This is also especially important if you use wire as an inserted bail for a pendant, it seems to hold when it comes out of the oven but a little gravity, along with the weight of the pendant, applied over time will allow the piece to work its way out, and the pendant will be lost.
I think that a little silver or gold wire adds something special to polymer jewelry work, especially when it's nicely twisted for a braided look, but a little extra care is needed to ensure that it's a quality addition and not a hidden booby trap for your customers.