I find that whether bi-wiring sounds better or not differs with the speaker and the cable. I would defend using bi-wiring with some speakers for sure. In general an improvement is possible by splitting the two halves of the crossover to locate the common earth at the amplifier rather than at the crossover. How much this is a benefit depends on the crossover design more than anything else. Minimalist crossovers benefit more from star-earthing, simply because other design parameters have been compromised in order to achieve the greater purity of using fewer and better components. Thiel crossovers are the oposite of that - heavily designed using many components to get phase and impedence curves as accurate as possible, and likely why Jim Thiel does not offer bi-wiring. Brit mini monitors with very minimalist crossovers seem to always sound better bi-wired.
Unfortunately, about a decade or so ago it became important to offer your speakers as bi-wirable because customers thought that was better, and so a number of designers added bi-wire connections even when there was no real reason to do this. What is more, people thought two sets of speaker cables must be better than one and so we got a whole lot of parameters being played with. It all added complexity to an already difficult problem - finding a great combo of amp, SC and speaker.
Some speakers do sound better bi-wired. My main point is that if you are going to single-wire a bi-wirable speaker then a few minutes with a screwdriver is cheaper and better. My second point was a bit broad, I admit. But I do know that some designers have put bi-wire connections on speakers when they don't actually believe bi-wiring is beneficial. That I find annoying. Good for Jim Thiel and a few others that don't stupidly follow the fashion for the sake of it.
One way of getting a good idea of how bad jumpers sound is to connect to the woofer and use jumpers to the tweeter, and then change to connecting to the tweeter and jumper to the woofer, and listen for the differences. When I have tried this I have usually heard significant differences, despite using pure silver Kimber jumpers. After that I decided to get the screwdriver out and do away with the jumpers altogether.