Absolute agreement. Nothing gets on my wick more than people attempting to sing a song which they haven't internalised and, to judge from the piece of paper they're holding, haven't even learnt the words.

IMO, learning a song is not just a case of learning the words and the tune. Far more than either, it's a case of learning what the song is about, and that can involve a close inspection of the textual nuances and hidden meanings. It can also involve understanding the song from a historical perspective. IE., what did it mean to the people who sang it below decks or whatever two centuries ago, and what does it then mean to me as a product of the 20th/21st century.

And yes, as someone who sings a lot of political songs and whose memory isn't what it was, I do frequently find I need a crib sheet simply because I'm dealing with a hot topic which won't wait. Under such circumstances, though, I can usually rely on sheer anger or euphoria or whatever to get me through.