Having less records is a privacy feature. Disappearing messages obviously isn't going to stop someone who is malicious from the get-go but if your friend leaves

Having less records is a privacy feature. Disappearing messages obviously isn't going to stop someone who is malicious from the get-go but if your friend leaves their phone unlocked or turns malicious later not having that history on the device is a big win. I agree that it should be advertised very clearly (maybe "Request that all members of the conversation delete their copies of the messages after this length of time") but even though not enforceable it is a privacy feature.