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.