Just to add to this debate. Led Zeppelin, Cream, et al, successfully 'borrowed' huge swathes of blues material without always accrediting the original performers. Carlos has always intertwined bits of music into his solos live as a sort of guessing game for the fans. Jeff Beck did it as well. Folk musicians have also done similar 'borrowings' in the past but nowadays they do seem to have the decency to acknowledge the song originators.

Plagiarism will always be live and kicking.....it's so much easier than writing ones own material.