What a clever idea! I have never thought of that conversion possibility before. It will definitely need a lot of reorganizing if you were to constantly change from a book shelf to a dress up cabinet though. Nevertheless, as long as there is storage space, there is bound to be more purposes that it could serve.
The conversion to a bookshelf is for when the child grows out of the "dress up" phase of her life. So that the piece can be used for a lifetime and not discarded after dress up time is no longer cool