When I started to play with Mycroft, I was more thinking about using a Sonos skill and found this one: Sonos controller skill - testing and feedback , which I tried but it did not support Spotify. So I created my own skill, for a number of reasons like: 1) it was fun to try, 2) I could add the commands that felt most natural to me.
I did discard using the existing Spotify skills, but maybe for wrong reasons, I realize now. 1) I thought the skill was limited to playing Spotify on the actual Mycroft device. 2) I thought that there was an issue with credentials that made the skill not “working” anymore. I believe there is still this issue, but that you can work around it by using your own credentials, like I do myself. I got a bit “scared” by the warnings in the readme of the skill and some forum/chat discussions.
So no real good reasons for your 2 first questions @goldyfruit :), except that I knew what I wanted to achieve (the exact commands I need when cooking dinner 
) and just pushed forward until I had the device I wanted, having fun in the process building a skill from scratch and discovering how the Mycroft, Spotify and Sonos ecosystems work.
You made me re-read about the existing Spotify skill, and adding support for Sonos there may be a good/better way forward for Mycroft in general, especially if we manage to solve the dev credentials issue which is not “general public” friendly.
Having a a general dedicated Sonos app would also be a great thing to have, I agree.