It would be extremely dull for Itachi's master plan to be to steal a power he already has....
I would quit watching Naruto.
The "true purpose" of Sharingan is the most likely motivation for sparing Sasuke and I'm betting it has something to do with Kyuubi (which will by default involve naruto) as Kyuubi suggests his own involvement with the Uchiha (through Madara).
This would explain Itachi's presence in a group like Akatsuki who's goal is to collect the Bijuu including Kyuubi (who is officially the last they will obtain).