Not only that, when dealing with trolls or members joining intent on causing trouble. The one thing they want to be able to see is if the staff are there online. Being cloaked from these type of members joining gives staff a distinct advantage in dealing with them (they don't know if your there or not) when logging in. This means you can monitor them in secret hidden away from view.
Just an idea to think about, not sure what others view on this one is?