I'm still holding out for Itachi just walking up calmly behind an exhausted Sasuke and saying well done, you passed the test. Now we can go fight Madara together, who was a the real guy responsible for killing our clan.

Maybe thats why he is fighting Sasuke with only the MS... he knows that Sasuke has been training with Oro and has faced many other types of jutsu, but only he can train in him in using the Sharingan. If Sasuke cannot meet the required standards, then he is not going to be able to fight Madara.

The only thing that complicates this theory is Madara's role... why is he still letting Sasuke live? Afterall, even if Itachi turns out to be en evil killer and Sasuke defeats him, Itachi will tell Sasuke that Madara was involved as well. That would mean Sasuke goes after Madara then. Does that mean Madara believes that even if that happens he will simply take Sasuke out, or do a Emperor Palpatine one and say join me by my side instead of your brother?