That was my origional idea to run a system similar to that but I dont want to lose the trunk space, and then there are more (longer) wires to run, plus you have to worry about running the wires and hoses from the rear hatch to the front discretely and then through the firewall, too much hassle for me.
Plus the external resivoir doesnt come with a low level light so if you want to install that (i would) then you have to buy that, and ru neven more wires, and then install a light somewhere to display that your tank is getting low. even more hassle.
The stock WW tank fits a gallon of fluid, has a built in low level sensor & light , and I get to run much much much shorter wires and hoses. BINGO I was sold.