There is a very similar thread on Narutobase: Itachi and Akatsuki
Slightly edited the posts here and there.
Then why did he let Itachi join the Akatsuki?
Tobi gained a lot from Itachi- help with tailed beasts, stalled Gaara rescue team, help with killing off the Uchiha clan while keeping his identity secret (Chapter 400,Page 9), let Sasuke be so that he can take care of Itachi as per his plan later only to use him as a pawn after Itachi dies, and once that's done he can attack Konoha without warning and seal the Kyuubi last.
Itachi was an eyesore for Tobi's plans and he never wanted Sasuke at first but when Nagato died he needed some one to sync with Gedo Mazo (Chapter 453, Page 17), so he recruied sasuke as he was short on members.
Itachi was happy since village was safe and also he was keeping eye on the Akatsuki. We have seen this as Konoha was attacked by Pain after Itachi died and not before that so it make sense.
Thread on Narutobase discussing why Itachi couldn't kill Tobi:Establishing why Itachi could never kill Obito once and for all
Why didn't he get rid of Itachi at least after they start to capture the tailed beasts?
Itachi was a Leaf's spy, but he was nevertheless a useful asset to Akatsuki. Add to this the fact that Itachi never had the chance to share any intel he gathered with Konoha, nor did he manage to interfere in Akatsuki's plans at any point and you get an ALMOST full picture. The last reason Tobi kept Itachi alive was because of Sasuke. As we know, Itachi wasn't willing to cooperate with Tobi really. But, thanks to Itachi's death by the hands of Sasuke, Tobi was able to persuade Sasuke into joining him. That's how he gained a powerful tool (because an ally is a wrong word here), especially after Sasuke got Eternal Mangekyo Sharingan. In other words, killing Itachi would be a complete waste. Even if Itachi was stronger than Obito, Obito could have get ridden of him at any point he wanted. He was pulling the strings behind the whole organization, ordering even Nagato around. If he wanted to, he could have simply ordered Nagato to do the job. As you can see, no matter the individual strength of Obito, he could have got ridden of Itachi anytime he wished. He didn't - not because he was too weak, but for the reasons stated a point above.