Did you forget that chapter? http://www.mangapanda.com/naruto/602/15
Obito's other half was crushed by the boulder. The part that was crushed was combined with a zetsu to keep him alive. Without it he litterally is half a man with a good share of internal organs crushed. When Madara grafted the parts onto him it kept him alive. The zetsu half though has a personality of its own and for years it has remained dormant inside him.
Did you forget this? http://www.mangapanda.com/naruto/654/9
Unlike the kids which are whole, these are litterally half illusions since zetsu can't entirely trick him. That is why half is an illusion and the other half is zetsu. In other words this was a battle of organ rejection. If Obito won his body would reject the zetsu and his original organs would be used. Since their basically crushed organs his death was immediate. If he had gone with the zetsus then he would have survived removal of the bijuus. That is why he is dead now.