I've always used bare wire, but am gradually moving away from it for a few reasons:
-12 gauge wire is so thick it doesn't seem to like to stay screwed down.
-cranking down the plastic nuts on the back of my receiver (trying to make sure the 12 gauge doesn't gradually loosen) makes me worry about breaking them.
-when would-be Axiomites come over for a demo I'll be able to swap speakers quickly and easily.

But mainly for #1 above.