there is also the possibility that the rings are a way for the akatski memebers to use the different biju's chakra. maybe the rings can channel their chakra so they can use it in a fight......
i also think that the rings are somehow used to keep all of the akatsuki members in check.
"Our deepest fear is not that we are inadequate.
Our deepest fear is that we are powerful beyond measure.
It is our light, not our darkness that most frightens us.
We ask ourselves, Who am I to be brilliant,
gorgeous, talented, fabulous?"
Actually, who are you not to be?