Its a good idea. Seems they only have it working with Spotify so far.
And at any moment, Spotify could decide they don't like it and it will no longer work with the service:
Partnerships: Mighty is intended to work with Spotify. While we've had discussions with Spotify about a potential partnership, we're not currently an official Spotify partner and Mighty has not been approved by Spotify.
For the time being, Mighty functions just like any other offline device that has the ability to store music. Your smartphone, for example, is defined by Spotify as an offline device. Terms of Use allow each Premium user to sync up to three offline devices with their account and each device can store up to 3,333 songs offline. Mighty is fully compliant with these terms.
Didn't those ship nearly a year ago?
I'm not s Spotify user...
It does say shipped June 2016. Surprised no one has copied it yet.