Except this would be infinitely more traceable and would still require quite a bit of development work, whereas bitcoin mining is really easy.

And using a botnet to do this offers no benefits from just running it on your own hardware. Unlike bitcoin mining, running spotify clients isn't resource intensive.

I suppose, but the thing with doing something like this on a botnet is you need to put that bit of spotify listening software on all those infected computers. Once spotify got their hands on the software they could come up with ways to defeat it.