Kakuzu probably doesn't have the skills Hidan has with his scythe. He's actually pretty good with it (especially when he uses his attached rope). Hidan also seems more "agile" to me, at least from their manga appearances. He was doing constant flips to avoid Shikamaru's shadow while successfully getting hits on Asuma at the same time. Even with superior speed (or so I presume) I don't think Kakuzu could pull it off quite the same way (again from manga appearances). Plus this would make Hidan pretty defenseless and would force him to rely on his secondary weapon for any offense.
Besides the idea of their teamwork is to get Hidan close to his opponent while Kakuzu attacks both of them with wide AoE jutsu.