Keith-
I was NOT talking about "spontaneity". I was talking about natural interaction and the removal of the mechanical interlocutor. I agree that for "environmental" portrait work, or "journalistic" portrait work, a large format camera is not going to be the appropriate tool. However, in a studio, on a tripod, with large format you can get out from behind the camera (there's no point to hiding behind it because once the film is loaded, you can't see through the lens anyway). With smaller roll-film formats, because they are prism finders, you have to keep your eye in the camera while working, especially when working hand-held, because a tiny movement can throw off your composition. You are creating an artificial barrier between subject and portraitist when there's a camera on your face. Perhaps because we're photographers, we forget that it isn't normal to have a conversation with a mechanical box.
Another advantage to portrait work with large format cameras is the depth-of-field control you have, and I'm not talking about applying movements here. How many of you have actually seen/used a traditional large-format portrait camera? Most of them have very minor if any rear standard movements, and none in front. You're not applying significant movements in order to control depth-of-field. The depth-of-field control I'm talking about comes from the focal lengths of lenses you're using - a 14" f5 lens has very shallow depth-of-field wide open, and even stopped down to render an entire face sharp, will nicely blur out the background creating that "3-D" effect people so like in portraits. Yes this can be done with smaller formats, but it requires exponentially faster lenses, slower films, and very small amounts of light (in the studio).
If you are working outdoors with natural light, it is a different equation.