The problem with forging messages is that the target of the forgery will claim that it's a forgery, and many (most?) people will believe him. Forgeries only convince people that *want* to be convinced. i.e. Those who already want to do something, and just need a public excuse. And once you forge something that is declaimed as a forgery, you get a reputation as someone who forges letters, and thus as a liar.
