Most likely he doesn't show his face because Tobi/Madara isn't useing his own body, he's useing someone else. Most likely he is useing Obito's body because if you read the Kakashi child hood manga you would have seen small glimps of Zetsu, which means he probaly have known Madara for a while before the Akatsuki and took Obito's seemingly dead body to Madara's dieing body. SO Im guessing that Madar used a perminant mind transfer jutsu to change bodies. If you think even more. The eyes are the same on Obito and Tobi/Madara, his face would be half crushed so he wheres a mask just showing is sharingan eye, the hair is the exact same. So all in all Tobi/Madara in my book, is actually useing Obito's body and that wont be revealed until Kakashi goes 1 on 1 with Tobi/Madara and he reveals himself after syaing "Long time no see Kakashi" and so on. and Kakashi will kill Madara, thus killing himself with the over use of the Sharingan dieing and seeing his father once more