The first time they re-cut the movie, it was necessary. It undid the butchery of the movie done by the studio. The third time was really just pulling out all the stops for fans. The differences between the directors cut and the final cut seem to be minor, mostly unnoticeable. The main reason I think the "final cut" exist is the digital restoration. The digital restoration that makes it look better than 99% of all the movies that you're likely to see today.
Imagine, if you will, Citizen Kane had the Blade Runner treatment. Imagine that the studio handed the film back and demanded a more upbeat movie. Imagine that in the cinema release for Citizen Kane, he narrated his story and that the identity of Rosebud was never revealed, or didn't even matter. Surely, it would be justifiable to have this version of Citizen Kane re-cut into the movie we have today? Similarly, Blade Runner was a movie that need a directors cut.
Now, in a world where people can have High Definition cinema in their homes, standard definition just isn't going to cut it. Especially for a movie such as Blade Runner, which, whether you enjoy the movie or not, was a masterpiece and a landmark moment in cinema for visuals and special effects. Digital restoration of these scenes was the least they could do. This justifies the third release.
Keep in mind, all of the 'new' parts added to subsequent versions of the film were done so from the original footage. No additional CG was added for the re-cuts and the only new footage recorded was to fix continuity errors. They didn't take this the 'star wars' route, and you have to respect them for that.