They're the best GP in multiplayer. Usually human player wars are very tightly contested, and you have to raze every enemy city. Unless you have a great artist from music, in which case you pop it in your newly captured city, ending resistance, providing 80% cultural defense, and can see FOUR tiles away, giving plenty of warning about surrounding cities. It usually breaks games.
Single player though... yeah they kinda suck. They CAN be poppted for radio though, if you want bombers. And great prophets can pop printing press.