Yeah, there are Force Ghosts and all that jazz for the good guys, but they are still dead. It's more of a happy afterlife. A Sith-resurrection would make them more like "A horrible thing from beyond the grave that should not be!" ala Frankenstien. Ups the drama because, y'know, sinning against the good and pure Force Ghost nature. Doesn't hurt the drama, it makes it more intense because, like the Terminator or The Thing, IT. CAN NOT DIE! AHHHHHH!!!! RUN!!! And so forth.