Magnus comes with the extra joints both as an alternitive to the black joints, which some may feel don't match his white paint job so good, and for added knee articulation.
Take a look at this series of photos from my recent
Revoltech Ultra Magnus Photo Gallery
Maximum knee articulation with regular revolver joint:
Swap regular joint for double revolver joint:
Maximum knee articulation with double revolver joint:
Prime does not come with extra joints these joints are specific to Magnus.
As far as I know you cant buy revolver joints seperatly. It would be great if you could, they would be fantastic for kitbashing.
Hope that answers your questions.
Yours AK