It isn't the crib sheet per se. It's using it as an alternative to learning the song at all at any level and, the real crime, flicking through your notes rather than listening to others.

Many people just cannot commit a song to memory, but a working knowledge of the song sounds far better than sight reading, and surely if it is for entertaining others, you owe your audience at least that much?

I usually have a piece of paper with first lines of set and key sigs blue tacked to my guitar.