Well, from my experience, the manual focus would be handy. Say if you shoot at Manual Focus mode, you use the back button to set initial focus, and shoot the nudibranch. As it craw toward the screen, you have to pick up the camera and move backward, or use the back button focus technique to refocus again. But if you have a manual focus knob, you can actually "pull focus" and smoothly adjust the focus. I found that using the back button to re-focus a subject that just moved would give the effect as if the camera is focus hunting.
Sometimes when you use the back button focus, it might just focus on the face (like the seahorse), but the eyes is not that focus, just the snout of the seahorse is sharp. This is when it would be nice to have the focus knob to bring up the enlarge screen and really focus on the eye especially with the help of peaking. I don't use pinpoint focus so this is the way I did when I test out the GH4.