The flat section is to lower top speeds so the driving is less dangerous because for a while there the speeds there were achieving were too high for natural human response time to be adequate to maneuver the cars.
In the first page of the forum, oil pan 4 stated that many people were achieving only small gains from such a big project as a flat belly pan, where as you could get the same percentage improvements from just an air dam (a less extensive project). A flat underbody
may be more efficient than all the bits that hang down from you car just in the wind, but if you want to see big, meaningful improvements, you really have to go for an airfoil shaped bottom, or dimple the sucker if you have to do a flat pan.
The passenger vehicle world is not the best place to look to for innovative designs. Their designs make money, and aren't always engineered to perfection. Some of the best cars out there, like the Tesla Roadster, and the Aptera, will never make money, but they are engineering to die for. Tesla is an electric car done properly, Aptera is real life eco car design.
freebeard, most cars aren't designed to be aerodynamic in the side direction
I think what you speak of is a good example of how to achieve overall good aero performance in the real world: a windy highway.