The didn't go to the planet for a person, they went for the secret of Genesis. Kruge separated the others so he could beam them up to his ship. And as far as Kruge was concerned, Spock was just some guy near death. Worthless to him. The others couldn't get to him because Kruge would shoot them, or worse, shoot Spock. Meanwhile, Kruge would have it out with Kirk. That way he'd be remembered as the badass dude who got the Genesis secret AND killed Kirk. Kruge thought he had what he wanted with Saavik, after all.