I think that too often today, people jump to conclusions about plagiarism and the first word that come's to their mouth is "sue!". I see nothing wrong with taking ideas and motifs from other composers and using it in your own compositions/arrangements, as long as it is not copying them exactly.
Personally, I think John Williams is a very good composer and I enjoy listening to the soundtracks from the movies he has worked on. From what I have read from previous posts, lets use Star Wars as an example again, George Lucas came to him with the pieces that he wanted the score to sound like, and Williams made a score in the way that Lucas wanted him to. Isn't that his job? It's good to come up with new ideas, but it is not completely necessary in his line of work, and as long as the director likes the result, everything is good. There was one example I listened to, the Stormtroopers example, I think, that was very similar, and I think this is pushing it a little bit too far.
Overall, I don't think its important to come up with new ideas, but if using old ones from someone else, they shouldn't be copied exactly and a new idea should be added onto the old. There are only so many rhythms and note patterns that can be used, there are bound to be similarities somewhere along the line.