By having the hatch ribs arc out and back to meet the floor you are providing room between the ribs and counter edge without sacrificing under counter space or needing to have a bunch of floor sticking out past the front edge of the counter.
The flatter or more tapered the hatch is, the further forward the counter's edge must be and the more floor you will have to knock your shins against when you reach. There are ways to mitigate this, such as slanting the under counter cabinets rearward to steal some of this wasted space back (complicating the cabinet building and not really solving the reach issue), or having that section of floor either swing away with the hatch or drop down like a tailgate (... but these are complex and more effort to seal well).
I'm pretty sure that the more teardrop shaped TD's are more aero than the boxier ones (15 deg down slope seems to be the magic number). Even if the square ones do incorporate some sort of spoiler and side spill plates, they are still a brick.
I built mine with a big bustle in the rear hatch because it allowed me to have nice 3 inch deep hatch ribs while still keeping the counter face frame close to the rear of the floor. This will also allow me to slide my cooler in lengthwise, providing a lot more under counter storage space w/o extending the overall length of the floor any more than necessary (9 ft 8 inches). Plus I like how it looks. Aero effect had very little to do with the decision, at least in my case.
My Build: The Poet Creek Express
Poet Creek Or Bust
Engineering the TLAR way - "That Looks About Right"TnTTT ORIGINAL 200A LANTERN CLUB = "The 200A Gang"Green Lantern Corpsmen