Alright you convinced me. Thanks for your time !

You didn't get it : adding a source package would mean building it at the installation and therefore it would just compile for the machine it's being installed on. Using an unoptimized version of OpenBLAS is completely useless or even counterproductive in a lot of cases.

Agree with @armandg and @mgrandl, I'd really appreciate a Mopidy package for my main computer. MariaDB is being on its way to the main repo, I can't see why a music player daemon wouldn't just because it's labeled as a server app.