I was looking for a gravity solution, not trying to analyze the picture for the landscape or anything like that. Well, the gravity does not always pull vertically down to the horizon... Anyway, using the virtual horizon is viable for static objects (except that it is behind menus in D3). The point is that it is not an option when shooting sports while looking through the viewfinder.
I shoot girls' soccer a lot - I use monopod - and sometimes I need to catch the action in the field and then I turn the camera so quickly that it is not horizontal when I press the shutter release button. I like to stand on the side line and then the girls run fast by me and my camera.
I might be standing too close sometimes. I had a close call the other evening. While having my eye on the viewfinder the ball hit a player in front of me and passed my head so close that I felt my hair moving!