To me it makes the movie weaker and stupider. It pretty much ruins the central redemptive act moment in the movie, when Roy shows mercy and saves his human enemy's life. It shows the human (Deckard) that maybe humanity has become less human (or: more inhuman) than these beings they treat like disposable tools.
Roy is a warrior, and he tried as hard as he could to win his fight - taking no prisoners and sparing no sacrifice. But now, with no chance left to win and nothing left to gain, he proves himself capable of mercy towards his inhumanly pitiless enemy. In terms of human qualities of mercy, compassion - Roy ultimately proves himself a better man than the actual man. "'More human than human' is our motto." - Dr. Eldon Tyrell. Indeed.
Absent that distance - Deckard for humanity, versus Roy as a replicant - if its just robot vs. robot, the film's climax becomes a juvenile M. Night Shyamalan "ooo we're all robots" "sci fi" "plot twist" - a twist that isn't remotely interesting, compared to what richness you lose in terms of the film's themes.
But even if you like the twist, or even if the themes mean nothing - just in terms of the film's story, Deckard being a replicant makes no sense. Based on what we're shown in the film, Deckard is considerably less durable, less fast and strong than any of the replicants. He's not remotely comparable, physically, to what Tyrell Corp has been putting out since four years ago. If he's a replicant, he's no Nexus 6. Or else he's a deliberately weak, slow and fragile Nexus 6. But why would the cops expect a substandard skin job to be able to air out Roy Batty, superhuman and of genius intellect? Plus his fearsome off-world kick-murder corps? "Oh we gave him jaded blade runner memories and a noir sensibility...that'll even the odds." Is this the sort of hunch cops in any hard-bitten universe will play?
If he's a human being, it makes damn good sense. The cops drafted him because, well, he's the motherfuckin' blade runner. Best there ever was at airing out skin jobs. He's got the magic, his killer instinct (retirer instinct?) has proven superior to any replicant ever made. But even though Deckard's the best, that special magic guy, it's also clear that blade running is a duty that human detectives are fit for. Or was Holden a replicant, too? Is Gaff a replicant? Maybe everybody's a replicant! "Ooooooooooo," but thanks, I don't think so. Being a blade runner is a shit job, but so is most police duty. Boss cop can assign a detective to do shit job. They bring in Deckard because he's known to be good at it. They expect him to deliver or die trying. Either way, that's a bulletproof plan. If he dies, he dies. Big deal, cops aren't any worse off. He was little people anyway, they'll assign whoever's left to the next lucky detective. But they have good reason to hope Deckard can retire at least a few of these rogues. It does make sense to bring Deckard in - if he's a human.
Now. Suppose Deckard's not a real, human, expert, lucky-touch blade runner. How could bringing in a replicant, especially a faulty, weak replicant, and tricking it into thinking it *IS* a blade runner help the cops? Worst case now is Deckard goes off the reservation, and you have another replicant running free! You've made your problem worse not better.
Deckard being human is the only way it makes sense the cops might make the call to let them both go - him and Rachael, both. Bryant and Gaff surely know he's utterly lost his taste for the business by now, but he's an ex-colleague. He held up his end. He got rid of the only replicants who stood to cause trouble. Bryant and Gaff look the other way because they know Rachael's no more a threat in Deckard's care than she was in Tyrell's.
If Deckard were a replicant, why would he even be allowed to leave the scene after Batty's death? It clearly wasn't so he could go kill Rachael.