[spookylukey]

There is no value in it because it is a false promise. If you give someone access to data, you have lost control over it, especially if it is by email - because not everyone uses Gmail. Many of those who do do not use the web client, and email clients are ultimately controlled by end users, even web based ones.

Only if you have complete end-to-end control over all the devices that everyone uses, including their brain, can you stop people from copying data.

If I have a file that I want to limit access to, I would never dream of sending it via e-mail. Some hosted document service which only shows parts of the file would be far preferable.

[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.