The main problem with Kamui at this current moment in time is the scenario, not the battle.
Against the seven swordsman, their are how many? seven. Kakashi can't use seven seperate Kamui's to defeat them all because he will drain his chakra on probably the third or forth swordsmans
Secondly, he's the general of the division. He needs to be in charge, thinking of tactics and so forth. Not on the frontline, firing kamui's and losing all his chakra. He wouldn't be able to be a general if he needs to be hospitalised, would he?
Thirdly, against the impure world resurrections, there is no certainty it would work because he would warp the body away and not the soul though if it were the soul as well, it would be dishonourable to the ninja who was revived. Kakashi will do the right thing and let their souls rest in piece, not send them to another dimension to rot for an eternity.