There have been some recent "dustups" concerning blogs which have had sections "borrowed" from others in them. In addition there have been continuing discussions of whether copying and altering other's work is appropriate. The general consensus is that this is a "bad thing." I'd like to argue the other side.
Plagiarism has a long and noble history. Shakespeare stole plot ideas as well as key pieces of dialog. Bach copied works of Vivaldi, in some cases adding instrumentation and in some cases reducing works to a single keyboard.
The original Three Penny Opera consists only of popular songs of the day with new words. The familiarity was part of the appeal. Mozart's arias were so popular that they were reworked for instruments as soon as the operas were staged. Beethoven wrote variations on themes of Mozart. Brahms used Haydn's themes, Rachmaninoff used Corelli and Chopin. Liszt and the other pianists of his age made careers out of arrangements of popular operas for piano.
In the visual arts there is a similar tradition which culminated in a sense in the work of Andy Warhol. Not content with copying the style or subjects of others, he used photo-mechanical methods to put actual reproductions of works into his creations.
In the movies we have the constant pattern of hits of the past being redone for a new age. Some popular stories have started as silent films and are still being recreated. Dracula is a good example.
In contemporary literature there have been several recent cases of verbatim paragraphs being lifted without attribution. I'm not interested in debating whether this is a copyright violation or "fair use". Copyright is designed to protect the monetary interests of publishers and (indirectly) authors and artists.
Suppose a mediocre book has passages lifted and put into a new work which is better than the original. That is certainly the case with Shakespeare. If people enjoy the new work then isn't this a valid aesthetic experience for them?
Much recent music consists of performers cutting, remixing and otherwise altering existing recorded music. How is this any different than the use of collage by visual artists? New software has extended this to the realm of video as well. Ren and Stimpy probably started the trend by adding a running commentary to existing images.
So, where is plagiarism not appropriate?
The first area is in scholarly publishing. By using the work of others without acknowledgment the chain of verification is broken. When research is published it is important that others know where the data comes from and who did the work so that the results can be reproduced, or otherwise double checked. If this attribution is abused than the scholarly work can potentially damage public knowledge.
A similar condition applies to news gathering. Without the ability to check sources news risks becoming propaganda. For news not only is plagiarism a danger, but so are anonymous attributions. This is a technique that is quite popular these days and has led to a general lowering of the public's opinion of the trustworthiness of the news media.
Because of increasingly restrictive copyright laws the cost (in time and money) of obtaining permission to use material has increased. In many cases copyright holders use their control to exercise censorship over material. In the past this was mostly the case when correspondence between well-known people was going to be published. One party would not allow this use because they didn't want their dirty laundry aired. These days restrictions are applied to items as trivial as a few words or notes from a popular song. This leads to plagiarism. Over control leads to breaking of the law.
The other issue is "artistic control". Many argue that allowing others to alter their work violates their "vision". I say that is just the way of the world. Once your "child" is out and about you no longer get to say what happens next. At most there should be a disclaimer that the derivative work is not "authorized". A good example of this issue concerns the case where commercial firms "clean up" movies and TV shows their customer's find objectionable. As long as the copy being modified is paid for what's the difference between me getting out the scissors and me hiring a firm to do the electronic equivalent on my copy? The objections are commercial, masquerading as artistic integrity.
The web has opened up new ways to expropriate other's work. A common one is to use an 'img' tag on a web page to incorporate an image from elsewhere. This happens (to me) all the time. Is it any different than cutting a picture out of magazine and pasting it into one's own diary and then sharing it with friends? We all are aware of the large number of music and video files being "shared". This is not plagiarism, this is theft.
The "intellectual property" interests have gotten so strong that a rational discussion of the balance between creativity and commerce does not seem possible. In fact current trends point toward preventing buyers from copying or altering copyrighted material even for their own use.
It is time for plagiarists to explain how art grows through imitation and adaptation, so far all the arguments have been from those interested in making a buck.