It has the potential to be his strongest move, but he is no Uchiha and he hasn't perfected it yet. Secondly as we saw with Deidara it isn't instantaneous, it actually takes some time and with Kakuzu's speed not to mention his reach plus the fact that unlike Deidara, Kakuzu isn't alone. Its just not a good idea

Plus even if he succeeds and takes out Kakuzu, he would be out of chakra and Hidan would still be there. Bottom line is, Kakashi's MS is not something he'll use except as a last resort and its not something to use if he's facing multiple opponents.