If anything; I think this gives people an impression that they can NSA proof their email-- when in fact they cannot.
The NSA is capturing, and archiving, the data directly from AT&T and Verizon's network. The fact it's set to expire on the OPs servers is of little relevance on theirs (NSA).
Cryptography is the only way to "NSA proof" anything; and only if you do it right.
As far as your comment on the headers, you're absolutely right. There will always be a record that you sent some message to your recipient at a particular time with a particular subject. However, the details of the message itself are never contained within the headers.
Also, with GhostMail, you already have the ability to "undo send". You can just go into your "sent" folder and view your message before your recipient views it, rendering the message unreadable.
'allows you to send "read only once" emails'
The more you advertise it the faster that will happen.