This is the best explanation that I've read yet of why a professional photographer would choose to shoot Micro Four Thirds instead of Full Frame: http://lindsaydobsonphotography.com/blog/micro-four-thirds-vs-full-frame/
The "low light" and "shallow depth of field" sections of her article, in particular, were very enlightening.
I'm also in the camp that doesn't understand why you would need anything bigger than Super-35 for video, which is THE accepted standard, and achievable with a Speed Booster on MFT. I would think you would only really need it if you're shooting for IMAX specifically, or if a client wants the razor-thin focus look all the time, like David described above.
The "low light" and "shallow depth of field" sections of her article, in particular, were very enlightening.
I'm also in the camp that doesn't understand why you would need anything bigger than Super-35 for video, which is THE accepted standard, and achievable with a Speed Booster on MFT. I would think you would only really need it if you're shooting for IMAX specifically, or if a client wants the razor-thin focus look all the time, like David described above.