I understand what you're saying, but I disagree.

The rooftop scene is far more moving without the voiceover. The audience is left to have those thoughts about why Deckard was saved without being prompted to by (frankly sub-par) Marlowe-esque narration. In any case, one possible reason - that Deckard himself is a replicant - is made clear in the final scene with the origami unicorn. It implies Gaff knew about Deckard's earlier dream of the unicorn (a scene missing in the studio cut). How would he know about it, unless Deckard was a replicant programmed with that dream? It's a brilliant, ambiguous, downbeat conclusion, supported by the various other hints throughout that Deckard is a replicant, and hinting that he and Rachel are being allowed to go free as a kind of controlled experiment. This is utterly ruined by the bolted-in conclusion in the studio cut (culled from outtakes intended for the opening scene of The Shining).

Yes, I love film noir (see here on that front, if you haven't already), and yes this film has a sci-fi noir aesthetic, but Blade Runner has always been a richer, darker, more profound experience without the voiceover.