USB vs S/PDIF

So is optical or coaxial really a higher fidelity interface than USB (with linear power supply, not bus power)? I mean by Slee design standards. I would be interested to read Graham's reasoning on this question. Is toslink/coax worth pursuing or is the grass not always greener? The purpose of digital audio is to be streamed to a receiver and USB seems to be the modern standard for transferring data between devices. Why aren't optical/coax used for devices like external hard drives? Is it just the convenience of USB having both power and signal in one cable rather than connecting two separate cables?

The Bitzie story starts here:

Thank you.

I'm no computer expert. All I know is isochronous USB audio is sent in "packets" consisting of ID and other clever preamble bits, a "snippet" of PCM audio, n-bits long, and an end-point. It seems to have "excess baggage." Somehow, the USB CODEC throws away the unwanted bits, leaving snatches of audio, which are shoved into a buffer, and emerge as a stream. In the case of data outputted, it is in S/PDIF form, but at logic levels.

The only thing "fed back" to the host is HID instructions such as volume control, which I "null." If the host sends a glitch, the glitch might screw the music around, or add noise, but I've not been able to hear it. Otherwise, it just keeps playing, and the "rules" are such that it does.

I once had a knackered PC, which while playing, stopped doing the other things I wanted it to do, and hung. It kept playing correctly until it blue-screened and shut down - for good.