He used him to make Sasuke stronger. Seeing as Itachi basically had clairvoyance, he knew Orochimaru wouldn't succeed with his plans.
Hey, you asked a question and I gave the correct answer to it, not my problem if you're going to deny it.
XD it's not the "correct" answer. It's "your opinion" on why Itachi didn't kill Orochimaru then. But nonetheless it is as valid as any other reason given by anyone else , though I don't think he intended for Sasuke to train under Orochimaru.
I think he just used that intellect to deduce that if Oro came after him and failed he would eventually seek his little brother-the only other surviving Uchiha (that we knew of at this point)
OT: In my opinion, there's a simple reason Itachi didn't kill Orochimaru when he could have. Itachi was a double agent. He infiltrated Akatuski to spy on their activities for Hiruzen.
Orochimaru too was a member of Akatsuki at the time. Killing Orochimaru then would have comprimised his position as a spy and Itachi would have been hunted by Pein/Obito and the rest for it. Just as they were going to kill orochimaru for leaving if things hadn't played out the way they did.