Dissect setup in this scene

[Tolin]

is negative fill really going to be that useful in such an open space? i mean if it was blocking out light from the water sure, but... it's what, blocking reflections from a hill a hundred feet away?

When you got very flat light, such as that during overcast, it means you have light coming from all directions (sort of). By placing a negative fill close to the subject you are effectively reducing the light from that direction, even if it's just by a little. This, as Moseph mentioned helps to create a less flat look.

[starcentral]

I guess this could easily amount to 1 or more stops of light difference. Take your light meter and meter it towards and away from light source (or overcast) and I guess that's the idea. It all makes very good sense indeed.