One of my main reasons I want such a DAC is exactly the same as for you Roger: I want something small to put on my desk which enables me to easily change the volume of my speakers, and indeed switch to headphones without having to crawl under my desk. Such a DAC does this.

It's funny, because about 10 minutes before I read your post I had the same thought: does my motherboard have optical out, and if so, wouldn't it be cheaper to just buy a bookshelf amp with optical in and without the DAC function, and so just keep on using the internal sound card, but optically?

As it turns out, yes, my pc's motherboard does support optical out (didn't even know that myself until I checked). And there are cheap bookshelf amps out there, like eg. the Orb Audio Booster or the Dayton Audio DTA120. (all between 90$ and $160). But it seems the headphones function, which is important to me, is not there then. I found one example that has both an optical in and headphones connection, the Denon DRA-100, but it's even more expensive than the DAC version (probably because it's also a network player).

This does ease my mind a bit about the lifespan of an external DAC : should at some point in the future the USB part become unsupported because it doesn't work anymore with a future version of Windows or something, I could always use it as a dumb external amp with optical input. When reading the user comments on Amazon, it seems this is the use case for most users anyway (because they connect it to their TV), disregarding the DAC features entirely - strangely enough.