I think madara went after rinnegan not just for power..........but to control the ten tails after he unseals him(if hes able to)and its other parts(nine tails and eight tails).............you see the eye of the 10 tailed had much resembelance to a sharingan and rinnegan fused..........it had tomoes and pattern ike the rinnegan..........maybe rinnegan helps in complete control of tailed beasts and jinchurikis(probably)..................maybe u need both sharingan and rinnegan to control JINCHUURIKIS...............madara maybe thought that naruto would be too tough for him so he stole the rinnegan