Thanks to a set of European trademarks, we can get a rare look at the way a peripheral's design has evolved along its development cycle. NeoGAFfer Shiggy found three trademarks for differentcasedesigns of the MotionPlus attachment. They appear in chronological order above.
It's basically the same throughout, except the bottom corners went from curving inward to curving outward! From the look of the final product, that "lip" is designed to help secure the new jacket in place. Alternate theory: the MotionPlus flares out to reduce the aerodynamic quality of the Wiimote, should it slip out of players' hands.