I do suspect that they're doing it simply because they knew it would cause a huge stir, and help them market the movie. As you say, there is no reason whatsoever to use Dean's digitally constructed image, then have another actor record the dialogue. It makes the movie massively more complicated, for no discernible reason.
Yes, it's a tragedy that James Dean only made three movies, and people will always wonder what might have been. But the thing is, this isn't it. It's just a macabre simulacrum of an actor who is locked in everyone's memories as the handsome, tragic tale of lost potential.
If someone wants to honour Dean and bring his name into modern cinema, then write a biopic. Get a great director and cast someone who can do the man justice. Though even that wouldn't be without controversy, because whatever interpretation the writer had of Dean's sexuality, they'd be criticised.
You're thinking too small, here. I want Audrey Hepburn, Grace Kelly and Marilyn Monroe in a sexy crime caper, about three grifting dames who take suckers for all they're worth.
Then they can resurrect River Phoenix and stick him in a movie with his brother, Joaquin, and his best friend, Keanu Reeves. It would be so much fun!