Remember when the Death God cut off the part of Oro's soul that contained his arms. What if Tobi is a combination of a fraction of Madara's soul and Zetsu's body.
Thats it! Thats the only frikking excuse everyone uses against that theory. Have you seen what other ninjas have done in the past, much crazier stuff. Minato had sealed a part of his chakra inside Naruto's seal, chakra is spiritual energy, spirit=soul. Also, if no one had killed Kisame's Zetsu clone, it would still be alive despite Kisame being dead. And its not that much of a stretch considering everything. You should reconsider. No one else is capable of summoning the Kyuubi.