I think that has to do the book, the movie adapts. In Do Androids Dream of Electric Sheep? all people are a bit like zombies, they only want to go away from earth or follow a strange religion like Deckards wife does. In a scene in the book, another Blade Runner suspects Deckard to be a replicant and makes a test with him. I think, Philip K. Dick wanted to show, that the difference between humans and machines isn't that big, as we think.
In the movie Ridley Scott seems to make a little different point. For me the movie said, that androids are able to the same emotions and empathy as humans, if they develop it (so states the theory, that humanity is developed). Deckard makes like Rachel and Roy the progress to humanity, although they take different directions.