Even if they downed Hidan, Unless it was with Kakashi's kamui, What's stopping Kakuzu stalling with his hearts and repairing any damage done to hidan?. Which would leave kakashi nothing left to deal with Kakuzu and shikamaru's team sure as hell couldn't deal with him.One would think they'd rather Hidan down than Kakuzu losing one out of his five hearts lol.......
If kakashi really wanted to put him down he would've use Kamui or Put raiton on his kunai or any other of his offensive techniques.
He wanted Hidan to go down but he wanted to save the majority of his chakra/techniques for Kakuzu and needed to keep a bit more back in case shikamaru's team got in trouble also, The plan was for Shikamaru to deal with him, allowing kakashi to focus most of his strength onto kakuzu without him wasting to much and tiring on Hidan.
Downing hidan is useless if kakuzu is there to repair the damage or if they haven't the strength to fight him after hidan's down