You need to remove the metal plates if you biwire or biamp. You only need the plates if you run one set of wires into the speakers. Also, not only do you defeat the purpose of running the speaker bi- if you don't remove them, you can damage the speakers.
binding posts
If your speakers have 2 pairs of binding posts each, connected by the ubiquitous metal plates, if biwiring or biamping should you remove the metal plates or doesn't it make any difference?