But $famous_player already has a jamendo-plugin!

Yes, there are already some plugins for Amarok & Co. But most of them just enable you to play music from Jamendo. This isn't what pyjama is meant to do (or at least not that alone): Jamendo is a huge free music community and one who likes Jamendo in most cases wants to discover new music and chat with other people about his/her likes and dislikes.

Pyjama wants to help you exploring free music. This is why it differs from most "regular" music players and this is why one might want to use it, even if Amarok & co comes with their own Jamendo plugin.