The logistics of it are different. With username + password, 2 people can have the same password. So you only need to warn someone if the username has already b

The logistics of it are different. With username + password, 2 people can have the same password. So you only need to warn someone if the username has already been taken. But in order to warn someone that a secret phrase has already been taken, you've just given them all the info they need to login as that other person. That being said, if you were to enforce that the first half (or some undisclosed portion) of the secret phrase couldn't be taken, then you'd be closer to comparing apples to apples.