From a storytelling standpoint, I think the irony of the machine teaching the man about humanity to be more powerful. Deckard operates like a robot, worn-out, unfeeling (very much the film noir hard-boiled detective) but through his experience with Roy is able to regain his emotions and perhaps begin to appreciate the Replicants as sentient beings (as Rachael is). Such a rooftop change of sentiment would also explain well why he decides to be "finished" with being a Blade Runner. Personal preference, of course, but I feel the end of the film has more meaning with Deckard as a man and not a Replicant.
Yeah, I just got done watching this movie for the first time in years, and the overall sense to me was that Deckard was human. I think fans have gotten a little too carried away over the years looking for tiny little "clues" that say otherwise, but the movie doesn't really seem to be hinting towards that at all. There might be a tiny bit of ambiguity about it (as Ford rightly believed there should be), but that's about all.
As for the movie itself... I have to say I'm still not all that crazy about it. I get that's it's film noir, but Ridley Scott seems a little TOO focused on the style and atmosphere here; there's not much of a story, and the movie doesn't really explore it's themes in any real or interesting way. We see a few glimpses of humanity and emotion in the replicants, and we're made to wonder who else could or couldn't be one... but that's about it.
I'm a big fan of this subject matter, but the movie just doesn't give me a whole lot to chew over or think about afterwards. Certainly nothing like what BSG has done with the idea. Or even TNG with Data.