Bad idea to do that! I dont know why but all i know is that there was a topic about this earlier and its not a good idea at all to delete the main os theme. If you wanna gain more memory delete all the factory ringtone/fonts/backgrounds.
Theme devs don't have access to all portions of the GUI through Plazmic, so every theme still draws some elements from the default theme. If you delete it, things will not work right.