@simplex I'm very much in favour of being able to sign messages so that their authenticity can be checked, because that allows users to agree upon firm pledges and commitments.
On the other hand, the cryptographic possibility that an unsigned message has been faked is integral to deniability, which is important to have in cases where the sender doesn't trust the recipient, but in practical terms, you should not actively encourage users to fake conversations, simply for reasons of social custom.