I've got spotify premium and I'm using spotify connect to cast audio from an android phone to a google chromecast audio
I also regularly listen from my desktop.
If spotify would allow me to stream the same audio to my chromecast audio and desktop player simeltaneously I'd have a ready built whole home audio system.
I think it would be amazing to be able to spotify connect to any and all devices on my account simeltaneously. a gigantic value add to spotify if they can master this.