Often they are bad. We have all seen how a last-minute change can derail a flick (coughcoughIAmLegendcoughcough). But in this particular case it apparently worked out pretty good for Rogue One (many, many spoilers in that link):
Star Wars: Rogue One has been in theaters for just about a month, and it is safe to say the movie is a huge success. Not only has the first ever live-action theatrically released Star Wars Anthology movie to take place outside of the Skywalker saga done very well at the box office, but it has been very well received by fans. One of the main reasons for that was the presence of Darth Vader, specifically his final scene. It turns out that scene wasn’t originally in the movie.
I’m a little surprised that something like that last scene wasn’t originally in the film, to be honest. I mean, you’d think that they would have shot it as early as they could. Out of sheer anticipation for Filming Something Neat…