The ability to change your pay rate without changing anything else about you profile or making any changes to other accounts like W2's etc., makes it very easy for me as an employer that wants flexibility when hiring/offering wages to my employees.
I dislike having all employers have access to this information but unfortunately its how things are now so we need something better if there ever will be one! If nothing more than just being able to offer different rates during times where demand might increase (for example right after xmas) would make them worth using even though they dont allow us our own private account.