Not every Rinnegan user was 'blessed' with the power of the Sage of the Six Paths, whose Rinnegan was enhanced because of the power of being Jinchuriki of the Perfect Juubi.
That said, the fact that the Rinnegan is the final evolution of the Sharingan interconnects it with it in my opinion. Sharingan is exceptional, especially as it is able to become the Rinnegan, maintaining all of its previous EMS Powers.
Those who acquired Rinnegan without first having it evolve (i.e. Nagato) benefit from the power of Rinnegan; which is indeed awesome. However, those whose Rinnegan had directly evolved from Sharingan during their use of the Dojutsu (i.e. Madara) benefits from all powers of Rinnegan as well as all powers of Eternal Mangekyo Sharingan.
The Sharingan, in theory, is an almost flawless Occular Power to possess, with massive potential of powering up, evolving, and thus adding to the users Occular arsenal (Obviously Rinnegan is a powerful Dojutsu as it is the Sharingans final form).