One of the greatest works of literature, the Iliad, was probably over half "plagiarized," and could not have been produced without plagiarism. It plagiarized _on principle_; once a thing has been well said one should not monkey around with it.
Of course the Iliad arose from an oral tradition, i.e. a highly fluid and easily reproducible medium (quite unlike the onerous business of setting type, etc., or of retyping everything one wants to copy. In other words, the oral tradition of Homer was very much like the internet tradition that is now developing. What should be of interest is the content and style of a given text; who cares who wrote it? Shakespeare also plagiarized shamelessly from his sources without giving credit. (He also usually improved on them, and an honest plagiarist should strive to make his/her plagiarism an improvement, at least in terms of immediate purposes, over its source.)
The campaign against plagiarism arose in the 17th century; it was part of print culture on the one hand and of growing commodification on the other hand, as well as on the absurd concerns with "self-expression" and "sincerity" that emerged in the 19th century. I would expect it to be a dead concern in another half century or so.
Carrol