Fair Use is a positive defense against charges of infringement, and one can only evoke Fair Use when you are charged with copyright infringement. The four components to Fair Use are: the nature of the work, the purpose of reproduction, the amount reproduced, and the effect on the copyright holder's wallet.
Now Fair Use is a good policy, but I stand on the side of the copyright holder because if somebody uses your work without permission there is no excuse they should either have to give you the credit or pay you the damages. The person being charged with infringement might say that they were not going to reproduce the copyrighted work, or that they did not know that by taking one, two, or three lines from an article or song that it was considered copyright infringement. But, in the end if you didn't write the work, cite the work.
