One of the most rational explanations that I've read with respect to the improvements caused by bi-wiring came from the XLO Electric web site (
http://www.xloelectric.com/technicalsupport.php#a10).
In a nutshell, they attribute any improvement from bi-wiring to the removal of the stamped, brass straps that are typically used to connect the high and low side of the crossover when not bi-wiring.