The only thing that scares me about shoots like this are the people themselves. They move, they blink, their tongues come out, they roll their eyes, they smile weird for a fraction of a second and there's not a thing you can do about it. So shooting only one frame would give me an impending sense of doom. I just did a shoot where this happened for several frames, it was uncanny. I've begun using a digital back-up for this reason only on shots that have to exist when I'm done. I'm up-front with the client about it too.