[vatueil]

There's no way to stop someone from copying the message if they really want to. That doesn't mean a system that prevents the recipient from accidentally leaking the message has no use.

There are plenty of scenarios where the sender may be confident that the recipient will not intentionally betray their trust but may not want to leave long-term traces behind (say, forwarding confidential documents to a family member). Rather than reminding them not to forward the email and to delete it ASAP, you could just use this feature instead.

Just because something doesn't can't cover all scenarios doesn't mean it's useless. Firefox's Private Browsing can't hide your activity from a determined eavesdropper, for example, but there are still plenty of ways in which it's useful.