I think that Ridley Scott added the part of Decker being a replicant so that the movie would have different meanings to everyone and cause great discussion about it. But to me, the unicorn left behind means one thing - that others have already said.
Gaff knows she's a replicant and knows that Decker is in love with her, and decides to spare her.
When Gaff says "It's to bad she won't live, but then again who does" means she only has four years of life and that Decker is becoming too infatuated with her. But then again, she dies, and no matter who he ends up with will eventually die too.
That's just me though. I also kind of hate the idea that he is a replicant. The movie to me is about man's ability to survive even against powerful things like replicants. I think director's cuts do this too much. It goes back to Star Wars. Who shot first, Han or Greedo... Harrison Ford's characters just need to be left alone, lol.
