Board Thread:Naruto Discussions/@comment-28462219-20160527161832/@comment-734582-20160615083040

TheFC12 wrote: It was for good reason as it was Hagoromo's eye to begin with Yea but Kishimoto made it that way.

Still, it's the power in eyes theme. Subtly, the Rinnegan shows that true power in the eyes cannot be unlocked if you also do not have a strong body. This is a theme shared with the Mangekyou: It loses it light because the Uchiha cannot sustain it's power. Only an Eternal can, but even then Obito proved it to be just a hack, as he could use his eyes without degrading, as was intended.