Is this a terrible thing? Why does content have to be original and unique to be good? Why can't content be similar, or even referential?

One of most notable example of a non-unique song, Aretha Franklin's Respect, is actually a cover, and the parts which are original to that particular rendition are quite repetitive. If we were to take out covers and samples from the body of music, we wouldn't have very much left.

From an entertainment perspective I might agree. However, music is a cultural expression (or should be). We should have stories to tell each other. Emotions more complicated than maudlin songs lamenting a lost love. Take away the content and music becomes a pretty screensaver. Interesting at first but non-engaging and without a message.