This is a good move, and it only covers a very specific case. Currently, you can apply to join a corp and as soon as the CEO accepts your application, you instantly join.

Let's say you have a one-man corp wardec you and you have the fellow on your friends list. He's not online and there's only one of him, he's less than a week old and your location agent says he's on the opposite side of the game. You figure it's safe to move your freighter around.

A battleship pilot sitting on a gate has previously applied to the one-man corp and the ceo is sitting in a conversation with him waiting for the go-ahead. As soon as the battleship pilot sees the freighter, he tells the CEO to accept him into the corp and he instantly "joins" the war effort. Surprise war targets appearing right in front of you is the exploit.

A similar thing with being kicked out of the corp can cause someone that's about to fire on you to get concorded. By the time they've fired at you, you're not in the warring corp any more. So this really has nothing to do with using the wardec system to grief or people joining/leaving corps during a war, it's about that specific exploit.