Adywan, remember that sometimes the ABSENCE of music can be just as powerful. Not having music during the ObiWan/Vader fight makes the moment feel more real because it has not been "movied up." In all the Star Wars films, despite the long and powerful score, Lucas has always found moments to leave the music out, making it that much more effective.
Personally, I think taking a part of the movie that has no music and adding it is an example of "doing something because I can." Again, consider that the music is left out of that scene FOR A REASON.
Also, consider how powerful the musical score is that is about to follow; when Luke sees Ben die and screams, it's one of the best musical moments in the film. Again, the absence of music preceeding it "clears the pallete" if you will and makes the cue more emotional.