Madara can use Sharingan powers while Rinnegan is active too. What is your point? It's only because it looks like a Sharingan ppl think using Sharin powers via Rinne is unique, and it isn't.In an earlier comment in this thread i already said Sasuke's Tomoe Rinnegam is better than the normal Rinnegan,but the user (Sasuke),is not stronger than Hogoromo.
Hogoromo is stronger but Sasuke's Tomoe Rinnegan is better.
It's obviously unique in power aswell due to having both Sharingan,and Rinnegan powers in one one eye,hence the appearance.