Kyrne I understand your point but let me explain my point in simple words:
Because of the password requirement the shout won't be enabled right? So it simply means that as an Admin I need to wait for my certain user to setup Shout with the password in order to message him/her.
What if they never setup their private messaging so I can't message them?
If you really want users to have their private messages secured and have control over it, you can give them an option to set up password for shout in their profile/settings or on the shout UI itself so they can decide if they want a password on their messages or not. but as default shout private messaging should be enabled for all the users.
Just like on facebook, twitter or any other social app.