Board Thread:Naruto Discussions/@comment-26488946-20150612205152/@comment-1251315-20150612213756

Nagato was nothing more than a pawn for Madara's future plans. They are not his eyes, so he can't use them properly as a result of not having them "naturally". Just like how Kakashi's Sharingan was a transplant and consumed more chakra, not could he use its abilities fully (the only known ability he showed was to be able to copy anyone's technique and later managed to use Kamui...).

I guess that Madara decided that Nagato was the most suited to carry out his plans.

Also, Rinnegan does not give Sharingan powers, so I don't know why you think he does. Had Nagato legitimately unlocked the Rinnegan himself, rather than having someone else's eyes, we probably wouldn't be having this discussion ;)