I seem to often use more tilt of the head (up or down) than can be easily done by adjusting the legs. Through habit, I like having the top of the tripod level. Hearing about where you have taken an 8x10, Drew, I think you deal with as much un-even ground as I do...but the camera just feels so much more stable by keeping the pod level. The camera's center of gravity stays centered.
And usually I need the extra height the head gives me. At 6'3", even the good height of a Reis A100 is not enough when photographing on sloped ground. It pains my back to be bent over or have to stand with knees bent for any length of time -- and I like spending a lot time under the darkcloth.
Another factor when photographing in the redwoods, I tend to push the legs many inches into the duff, trying to get to solid ground. Repositioning the legs to change the angle of the camera is not convienent if changing the leg length will not do the job.
But if I ever photograph in the flats, I should give headless photography a try. I am just so use to being able to change the view by a degree of two in any direction, and this includes panning with the A250 head, that moving, or changing the length of, the legs seems awkward. Reis legs do not easily change length -- especially with a Zone VI 8x10 and a Fuji W 300/5.6 sitting on top of it...especially when trying to fine tune the image on the GG at the same time.
Vaughn