Why USB to SPDIF and not optical?

There is no "valid" reason. It's a question of well the solution is implemented on a unit by unit basis.

Apple has hung its hat on optical and most everyone seems very happy with the results they are getting from the Mini using the best Toslink cable they can buy.

FWIW, a good SPDIF cable - and this is definitely one of those cases where there are real differences between cables - is both more expensive and shorter then either Toslink or USB.

So you are saying there would be little reason to spend $500 on the Bel Canto USB link, for example. Performance enhancement likely wouldn't be that noticeable? I thought I might need a similar device because the soundcard in the Mini is lacking?

The Toslink out from the Mac Mini has very poor jitter performance. It would be worth comparing that to USB from the Mac Mini directly to the Bel Canto DAC3, which has a USB input.

An inexpensive USB to SPDIF converter, such as the HagUsb, may or may not be an improvement over Toslink. Bel Canto's USB add-on gets you 24/96 via USB and, I expect, also gets you much better performance across the board. A device like the Empirical Audio Offramp takes it up another notch or two and leaves Toslink in the dust.

Okay i have the same confusion... i also have a DAC3...which has spdif AND USB inputs.. and i read on the Bel Canto site...

"The USB Link 24/96 includes a Stereovox XV2 BNC/BNC cable with RCA adaptor to feed your audiophile DAC of choice – most likely a Bel Canto DAC3!"

So... all i can derive from this is that the device must just be a usb based sound card?

I've seen first hand gains from going other than USB into the DAC3. Each step up by way of interconnect seems to gain a darker backdrop, and more ease and detail. Depth as well. the best connection thus far that I've used is via the BNC input jack on the BC DAC3 through a Stereovox xv2 IC.

Via coax things get better over TOS by way of jitter reduction and sound stage acquisition. The DAC3 seems to catch the info and reproduce it with a stronger grasp.

With even the use of an adapter, things are better using coax into BNC... especially by way of ambient recall or retreval of the orig recording venue.

these are incremental increases, yet descernable without too much care in listening. Most noticeable is the jump from USB to coax. IMO.

If of course your ripped info was encoded at more than 16/44.1, then even TOS WILL BE A DECENT LEVEL OF improved refinement over it's USB imput.

These items I've noted too might also be, in part, the result of the jitter or dejittering characteristics of the cable interface being used.

I found it impossible to find a maker of all 3 sorts of cabling, USB COAX, & BNC of the same level of quality to experiment with, so keep that in mind too.

Given all the cables I used are quite modestly priced, I'd submit my findings seem valid enough. No uberexpensive wires have yet to remain here, though several were tried.... some upwards of $750 MSRP per 1M.

Perhaps, having another sound card installed with your choice of interface (coax or BNC) would be an option, and possibly save you on many accounts.

Good luck