I have a problem with this too. If the family sharing would be more flexible with the payment options or who pays what we would be all over Apple Music.
Our situation is like this: We have active family sharing in my family and I am the Master of the Universe and my wife is the other adult. We have only small kids so no one else in our Apple Family. So we share my credit card. That is all cool.
But with Apple Music we would like to "extend" our family to include my dad and my brother. We are all adults and live on our own. And my dad would be cool to pay for Apple Music for all of us. But he is for sure not cool to pay for my apps and my wifes Hayday diamonds and whatever my brother decides to buy.
I think our extended family is still a family, but Apples strict 1 credit card policy really makes it impossible us (or at least very difficult). Or maybe some one have a solution for us that I'm not aware of?
I have an unlimited Spotify accound (so I pay 5€/month) and my brother pays for unlimited (10€).. So I guess it seems that we need to wait for Spotify to give a better Family deal and just skip Apple Music.