since S/PDIF is consumer class people are used to connect S/PDIF devices
and I assume everything will work without the need to care about
S/PDIF usually isn't used for audio production, this might explain why
people tend to forget to set up the clock source. AFAIK there's no need
to care about it, when using stand alone consumer devices.
I suspect that sync very often is the culprit, when S/PDIF used with
prosumer or pro-audio devices doesn't work.
Since I'm not using S/PDIF I ask somebody using it, to write a Linux
audio Wiki that mentions to set up the master clock manually.