Yeah, I think the only instance where this kind of think is justifiable is when making a sequel to something and an actor playing an important part in the previous film is dead (or really old ;-) .) I really liked the use of CG Arnold in the new terminator or Marlon Brando in Superman Returns. And it is now basically the only way to make the Thrawn trilogy into a film trilogy, which I must admit, I'd love to see if someone really good was hired to adopt the books to scripts. They could use the original actors through performance capture.