Spotify denied my application for commercial usage of their songs with Walkout Song DJ. I did not understand their exact reasons behind the denial, so I sent them an appeal request with further explanation of how their songs would be used in the app. It took them 8 months to respond with the denial so I am not hopeful that I will hear anything soon or maybe ever.
Sad part is that I have the integration with their sdk developed and it works very well
Perhaps you could send them feedback that you would like to see that support. If enough users contact them, they may see some value in the combination.