Board Thread:Naruto Discussions/@comment-25212736-20140722185642/@comment-24531362-20140723064708

Yalocalsupahero wrote: The only issue with that is it would have given Black Zetsu away. He spent years pretending to be Madara's Will. Which makes it seem unlikely he could use abilities not avalible to Madara or a Senju Wood clone body, without arousing Madara's suspicions..

Zetsu knew about the powers of Kamui, specifically the whole portal thing. Madara knew about it too, and knew about Zetsu-Obito possessing the other eye. Zetsu opening the portal would therefore not be unexpected, nor would it surprise Madara. In fact, it would logically seem to suit Madara's purposes, so the Uchiha wouldn't think twice.