Sora Nakachi is the most expensive Private Maid in the base set for a few reasons. Not only is she worth 2 VP , but her ability can be extremely powerful. If you have an annoying opponent who likes to throw Bad Habits around, employing Sora will ensure that they leave you alone. You could also be a jerk and play favorites, moving Bad Habits and Illnesses from one opponent to another. Of course, it only takes one player to purchase an Illness to disable Sora, and that that point you'll have to either cure her with a '3 Love ' card, cure her with Claire Saint-Juste, or just move on and employ a new Private Maid.
In a game with less aggressive opponents, Sora is mostly worth purchasing for her VP, as well as to advance the Private Maid stack.