Possibly it might be to do with speed; it might be quicker to throw a series of knees MT/MMA style than to throw a series of knees Karate style. As you say, you can't count on a single knee doing the trick, so perhaps it might be more with the idea of a cumulative effect?? I don't know for sure. Do you have a video example of karate knee?
"Let your food be your medicine, and your medicine be your food"