I've applied for a commercial Spotify API key. They let you start using it right away. I haven't heard back from them (it's been a couple weeks). It's possible they'll decide not to approve my use case, in which case I'd have to drop spotify support.

I think they mainly want to prevent people from competing with them; I'm not sure if they care too much about charging you money for it (I guess they probably would if you were making a lot of money).

Also, thanks for your work on Mopidy!