That's right, my example was off because film is 'usually' shot 180 degrees. So 30 FPS has an effective shutter of 1/60", and 120Hz open shutter needs a 2-frame

That's right, my example was off because film is 'usually' shot 180 degrees. So 30 FPS has an effective shutter of 1/60", and 120Hz open shutter needs a 2-frame sliding window to match that look. Or use a 3-frame window for 1/40", a more blurry film (1/48") look. There's power in being able to make that choice when editing. What I'm not sure about is how similar merging two images together will be compared to doing it natively with the camera. I'm guessing it will look slightly different, but I'll find out when I try it.