I don’t use Spotify but I know a lot of people that do - even those that like high quality streaming and playback have other needs too - on the road, family, historic choices, etc etc. Spotify would be good. And Qobuz and everything else out there. Choice is good. A packed ‘services’ tab would be great. 
The tricky part for any developers is interfacing with different companies and their APIs, as even though the framework’s probably largely the same, they won’t be identical and so still have to be modified and tested. It’s definitely a lot of work but probably not as much as doing the initial one. But Roon don’t use ‘standard’ APIs from what I gather so it requires not only time and development, but agreement from the service provider to give Roon a lot more access to their systems/data. Perhaps Spotify wouldn’t be interested in that, who knows.
Maybe Roon will come up with a different way of doing things in the future, or other providers will partner in the way Tidal did, but I sense there’s only going to be one service in that services tab for some time…