The fact that the Cultists know this means that they will just target someone else- in practice it's not actually different from a Townie that doesn't get targeted at all through the game (which can and has happened).
Not only that but it's tecnically not a permanent +1 to town ranks- because we don't know if we can trust you and we can't prove that.
Even if you are an uncovertible Town member you're the only one who knows that- this makes it less useful then if we all could rally around a publicly confirmed one.
I mean, it's not like I know it's wrong. I just don't trust it.