You mention that Itachi wouldn't do somethin to truly hurt someone and that this is a fact.....ask how his clan feels about this. Also, ask the Jinjuriki he assisted in capturing. Also ask the Jinjuriki he was indirectly responsible for by recruiting Deidara.
You mention that Itachi was never serious in battle....however for about 97% of his appearances he is dead serious...he just doesn't always display his full power. Ask Kakashi,Kurenai, and Sasuke during part I....he went pretty serious on them...even though he didn't go all out. I think his least serious moment was when he asked Naruto how he survived Chibaku Tensei if it kills who is caught in it.
On that note, you mention Itachi never fought at his full power, he did fight at his full power against Nagato; however, he had two teammates as well as Nagato being Idle due to Kabuto's take over.
Finally you mention he wasn't serious against Kabuto.....however he was. However, due to Kabuto's power, Itachi was not able to kill him outright. Also some notes in this battle.
1. Itachi was caught in Kabuto's genjutsu. He needed Sasuke to break out. Also, this was before Izanami was complete.
2. Itachi was cut in half by Kabuto before Izansmi was complete, Sasuke was needed to save him.
3. Kabuto didn't have access to his full potential either (every Edo Tensei he had deployed).
To answer your last question....for three reasons.
1. Itachi possessed an incredible amount of information that would make Akatsuki's goals far harder to accomplish if being actively hunted.
2. Itachi was aware of Obito's existence as Madara (heart of Akatsuki) and therefore any action taken on Konoha or anyone he deemed worth protecting would be met with Itachi knowing his target.
3. Itachi is an extremely powerful and badass combatant.....it would add yet another powerful enemy to the enemy's reserves. In addition it would give more enemies more information....and could bring them closer to utilizing more effective counter operations as well as figuring out more efficient methods to hinder Obito's goals.
Now that I am done Knit-Picking like a douchebag....I shall follow up with hard to argue opinions.
1. Itachi was an amazing person....despite taking a rash course of action, it was done with pure intent and love for his brother.
2. Unlike other possible people, Itachi planned for a way to atone for his actions even though they were operationally justified.
3. He was a pacifist (awesome)
4. He was a badass
5. While his stop to Edo tensei seemed to only stop a small number of lesser Edo's...though Chiyo and Kimmimaru were good stops (still...Naruto was closing in) it still easily saved countless lives.
6. Despite being criminalized and demonized, he stuck to his Nindo as a true ninja and did what he could.
7. Played a large role in helping save Sasuke from himself.
8. Was extremely intelligent and perceptive.
And that is what I have to say about that!