I do think Tobi is Obito. I believe this is one of Danzo's Secret that is shared with Orochimaru and Kabuto. I believe when Obito was injured to the point of death he was found by Zetsu and put into Sasori and Orochimaru care where I believe in one of Orochimaru's weird experiments he put Marada into Obito's body sucess, only to think it was a complete failure. I believe knows this and is afraid of how much Kabuto know's about his destructive past. aka Uchiha Massacre and Marada's Revial.