Didn't Black Zetsu died by Sasuke? I could have sworn that happened.
No, when Madara implanted Hashirama's DNA into Tobi in order to prevent him from dying he created a remote link between his chakra and said DNA. By injecting his own essence into it like in #636 he was able to assume control over that half of Tobi's body. That's how Black Zetsu came to be but the original one who was partnered with White Zetsu and the Zetsu flesh utilised in preserving Tobi are different from each other.
In Tobi's situation the right side of his body was the second creation of Madara's to be transformed into an Uchiha/Yin based replica of Hashirama's DNA. Zetsu exists independently of Tobi though.
I remember a scene that he just killed it and where the white Zetsu confirmed that he killed the original.
Sasuke killed the original White Zetsu. The last we saw of Black Zetsu he was only incapacitated.
Remember, Zetsu wasn't a fighter, more like a scout.
True, but Karin isn't exactly a fighter either and she would classify as a henchman since she aligns herself with Orochimaru and Sasuke.
Moreover, it's not like Zetsu is incapable of managing himself in combat, the black half at least. At the beginning of the war he seemed to be holding his own against Mei and the protection squad. He may not be the most powerful member of Akatsuki but he is capable of fighting against powerful shinobi.
Just the fact that we still have the whole Orochimaru and others already makes it confirmed for one more arc. If that's not enough, Naruto is at the limit. Unless Kakashi got senzu bean.
I half-expect that next arc to play out like the Sasuke Retrieval mission with several of the Rookies participating in individual fights against Taka. All of it culminating in a climactic battle between Naruto/Sasuke/Kakashi and Orochimaru.