If this works as expected (and i plan on trying it out this weekend) it would lead the way to being able to add Spotify Connect devices to existing Stereo systems cheaply (a Raspberry Pi and possibly an external sound card)... Do the Pis still have crappy Stereo Audio? i remember the first (and possible second) gen ones being bad... not sure about the newer ones...
To repurpose your old stereo with a raspberry today, just run https://github.com/mikebrady/shairport-sync on it :-)
This way you can send any audio from laptop/phone to the stereo over lan.
Not sure why you would want to limit yourself to the service of a single provider. (What value does Spotify Connect add, if any?)