The answer depends on the speakers. If the cross-over design is near optimal then there is no significant benefit from bi-wiring. But for many speakers, either the speaker drivers themselves, or the execution of the cross-over, provide quite different loads on the amplifier, or even cause a significant back-EMF from one driver to affect another. The Martin-Logan hybrid speakers are a good example where the difference in the characteristics of the driver elements means bi-wiring is valuable. A further example can be seen in many British mini-monitors where the cross-overs are kept very minimalist in the interests of resolution and immediacy, but at the expense of good driver impedence matching. At the other extreme, Thiel speakers have very sophisticated cross-overs and Jim Thiel claims that as a result there are no benefits from bi-wiring. Even though the electrical issues seem insignificant, the effect of bi-wiring is to star-earth the separate drivers at the amp. Without bi-wiring you are star-earthing at the speaker terminals (or somewhere in the cross-over itself in single-wired speakers like Thiels). Obviously star-earthing at the amp is better, but some would argue the very low resistance of the speaker cable makes the difference irrelevant. Nevertheless I have seen calculations that show even the very low resistance of a speaker cable is enough to create distortion through the lack of star-earthing at the amp. So to make a long story short - there is no general rule. Some speakers really do need to be bi-wired, whereas others do not. The problem with bi-wiring of course is that you have to spend twice as much on speaker cable than you otherwise would have, and you really should get as good a speaker cable as you can - most people underspend in this area.
