If I'm understanding the exploit correctly. An attacker could be doing a mitm on the server, and be able to determine if it is a first time connection. Then if it is a first time connection they could intercept and pass along their own key to the client. Since it's the clients first connection they would probably just accept it. Without this technique, the attacker would have to intercept all the traffic and would then be more likely to get caught.

I think it would probably be better if there was a standard for an out of band way to validate the servers public key. Like store a hash in a DNS txt record or something.