He is a mastermind. So, he made himself look a less danger in the battle and let the alliance handle Obito waiting for the moment of Juubi's extraction. Once that happens, black Zetsu emerges and controls Obito to use Rinne Tensie on Madara. All according to plan.
Or, he wanted to take Hashirama's SM and fight Obito himself.
He stated he was going to take Hashirama's Senjutsu and weaken Obito himself so that he could switch with him. Naruto and Sasuke did it before Madara could get around to it though.