Why do you think Bi-Wiring improves the sound ?

It doesn't make a difference.  And the whole diagonal wiring thing is another nonsensical myth.  I implore you to try it for yourselves, but make sure no alcohol is consumed between the time you start the wiring experiment and the time you sit down to listen.  

Richard Vandersteen insists his speakers are bi-wired per his design, so I bi-wire mine, and have never used jumpers on my Vandy’s. Not sure what the big deal is. I’m certainly not going win an argument on the subject with Richard, and have never seen anyone who has when confronted with the subject.

Now, if your speakers only have one set of binding posts, I see no good reason to add them if that is how they were designed.

The only benefit of bi‐wiring is the increased speaker wire gauge equivalent you get running two sets of wires. Running a twin set of wire is equivalent to increasing the speaker wire guage by 3. For example, running two 14 guage wires is equivalent to running a single 11 guage wire. I have tried both routes and I would say If the cost is minimal, there is no harm in running a bi‐wire setup. I wouldn't go to the trouble of adding dual binding posts to a set of speakers though to accomplish this.

I bi-wire because the speakers are so designed & mfr encourages &  already had the wires from last setup. Adding a second pair of binding posts just to enable bi-wiring strikes me as nutty, as others have said. 
Nobody thinks bi-wiring is harmful, so if you've got the stuff already, go for it.

No! Bi-wiring does NOT improve the sound if your speaker gauge is sufficient to the situation. As mentioned doubling the gauge of the wire “could” help... but maybe not. It is probably the least expensive “Audiophile” tweak you can try but do try and let the rest of us know it it actually works for you! I’m willing to bet it will make no difference in the actual sound. 
** actual results may vary :-)