Zetsu's cloning ability is not a normal transformation. He even has the chakra of the person he transforms into but obviously Tobi can't allow Zetsu to play Madara...
Kisame would have no way to tell who that guy is. Even sensing his chakra with Samehada and it would do nothing.
It's evident that he really was suppose to look similar to Madara, else him removing his mask to show Sasuke his face makes no sense, as not only would Sasuke clearly know he wasn't Madara, but would be able to counter a genjutsu from Obito.
What if when he wanted to show his face and look like Madara he used a Zetsu clone transformed into Madara (he defenetly has Madara's chakra).
Nobody could ever figure out that a zetsu clone is not the real Madara. Those damn thing replicate the original just about down to the last detail.
So you're settling for the Zetsu morph as well? Now that I think of it Tobi revealed his face to Sasuke and older Kisame pretty much around the same time.
What Kisame saw was the downfall of the series...
This is it. This is the best answer and fits perfectly into everything. If Kishi didn't think of this, he shold just adopt this idea! Somebody tell him this.
---------- Post added at 03:01 AM ---------- Previous post was at 02:52 AM ----------
Although I doubt that there was no Hyuuga who knew what Obito looked like, I can agree that (as Obito did sneak into Konoha to summon the Kyuubi [most people didn't even know that it had been summoned]) it is likely no Hyuuga (from that generation) saw Obito with his mask on...