The main reason why saloons / coupes do not have rear wipers is because they are difficult to mount as the bootlid moves independantly to the glass where on hatchbacks and estates they do not.
Whilst it's true that the flatter (more horizontal) glass gets less shit stuck to it a wiper would still be useful reversing after the car has been stood in the rain. They don't fit them because it's difficult.
Current: 2013 F10 520d M-Sport, Professional Media, Visibility Pack, Reversing Camera, DAB
Previous: 2011 F10 520d M-Sport, 2008 320i M-Sport Coupe, 2002 325i, 2001 318i valvetronic, 1998 318i, 1996 525i, 1990 Porsche 944S2