Is S/PDIF worth using?
Solution 1:
The answer is, of course, it depends. The advantage of S/PDIF (both coax and optical) is that it carries a digital signal down the line. This is essential when you're doing things like sending a DTS or Dolby Digital (AC3) signal to a receiver which can decode those signals. Often receivers have better digital to analog converters than on board audio controllers. Additionally, if your speakers are far away, an S/PDIF allows you to carry 5.1 over just one cable.
However, if you're just piping audio from your computer to a set of inexpensive desktop surround speakers, you're not going to hear much of a difference in most cases. If you can provide more details about your setup, it'll be easier to answer if S/PDIF is right for you.
Solution 2:
While most tuners don't have coax S/PDIF support, they almost all have optical S/PDIF support. The good news there is that the signal is exactly the same. You can pick up a converter widget from Amazon for about 15 bucks that'll turn coax into optical (or vice versa) - which means you can happily pipe the motherboard coax signal into a new optical tuner.
This is, in fact, the exact setup I'm running now. Love it.
So yes, I'd say it was worth it. Surround sound off the PC with only one cable is pretty fantastic.