Bi-wiring reduces inter-modulation distortion (IMD). Whether or not you can hear the difference depends on the magnitude which in turn depends on the current, cable and speaker characteristics. I've tried it, couldn't hear the difference at any level, but it doesn't mean it wasn't there or that others can't hear it on their systems.

House of the Rising Sone
Out in the mid or far field
Dedicated mid-woofers are over-rated