From memory cars with rear wipers used to park in the upright position (which is the commonsense way to leave the blade as stiggysawdust says) - I'm guessing some health and safety numpty decided that it may obscure rear view from car, and we ended up with the no brainer horizontal wiper blade.
leaving the blade vertical also keeps a lot of mid-day sun off it - and the sun is what perishes the rubber and cracks it.
Another mod worth doing is to turn the rear washer jet to spray 45deg across the screen (it is set at factory to spray straight down the screen and most of the washer fluid misses the wiper and goes onto rear door / bumper) spraying across the screen leaves more of the liquid on the screen for the wiper to use.