[jdietrich]

Ground loops. Stages are very electromagnetically noisy, mainly because of lighting dimmers. If you connect a mains-powered computer to a mains-powered master keyboard via a USB cable, you've just created a big antenna for all of that RF noise. That noise will travel through the ground plane into your USB audio interface.

In that environment, isolation is a constant concern. Connections that are inherently isolated make fault-finding far more straightforward and obviate the need for external isolation boxes.

[danellis]

Must be a solved problem, since USB is ubiquitous.

[zero_iq]

Only solved in the sense that you can buy additional hardware for ground loop isolation for USB and audio connections. Ground loops and RF interference are a persistent and recurring problem in pretty much all USB audio setups, except perhaps when you're solely using relatively high-end hardware that has been designed specifically for audio (cables included). Without isolation, all it takes is one bad component to ruin the whole setup.

For even a simple USB audio interface setup connected to a single audio source I would recommend isolation and balanced audio cables.

[ATsch]

It's a "solved problem" in the sense that it doesn't matter for the use cases USB was designed for. Most people are using USB to connect a few devices at most and over short distances, so these effects are negledgible. But that is not true for stage audio.

[SeanLuke]

Not even remotely.