Itachi and Kisame's duo was really formidable indeed. But imagine the whole Akatsuki, all members together. Sasori alone with his ultimate 100 puppets was enough to destroy a whole nation, while Konan with billions of explosive papers, and Pain's Shinra Tensei that destroyed Konoha... All ninjas from all 5 nations must combine their power in order to stop them all. They are too strong, that's why Kishimoto-sensei had to divided them into groups of two
Itachi and Kisame's duo was really formidable indeed. But imagine the whole Akatsuki, all members together. Sasori alone with his ultimate 100 puppets was enough to destroy a whole nation, while Konan with billions of explosive papers, and Pain's Shinra Tensei that destroyed Konoha... All ninjas from all 5 nations must combine their power in order to stop them all. They are too strong, that's why Kishimoto-sensei had to divided them into groups of two