I'm assuming you have a threaded anchor embeded in the wall panel. If so, it's about keeping the threads clean. Coil rod doesn't mind a bit of grit in the threaded anchor. It's also usually cheaper than threaded rod and most precasters will have more of it than they will threaded rod.
That said, if you're using epoxy or grout to set the rod then I'd agree that rebar is easier and better than coil rod.
Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries