24fps lets you focus attention with motion blur (tracking shot on two characters walking, they will be in sharp focus, while everything else will be blurred but still convey movement, etc.). It's a similar tool beyond depth of field. However, a 60hz movie could just have 30hz portions. Stuff like panning cameras over a landscape looks terrible at 24hz. I don't know how you could combine the two without making things feel too much like changing between technology all the time during the movie though.