Adding functionality for "Proof Approval" would be huge! And would allow the process to remain within Adobe as opposed to having to go outside Adobe products. Turning on this functionality when needed, allowing for proof to be sent that can be officially approved within AI that:

a. Doesn't allow the user the ability to download the PDF and steal the artwork.

b. Sends confirmation that the Proof was Approved or Not Approved with feedback. This can help streamline the approval process ensuring everyone is aware when a proof is sent, and status changes (Pending, Approved, Not Approved, etc.).

c. Saves the history of approval status and changes, so that the proof approval date, time, and by whom can be seen and referenced. This is for records and potential disputes.

d. Possibly a "Compress and Convert Images to Raster" option that could allow the person approving the proof to download the proof marked "Approved" for their records. This again protects from clients stealing artwork.