It's possible to inherit directly from the class Plugin. But by doing that, the only thing happens is that the object is created, nothing more. You may need this to create a thread in the background an connect it to the plugin interface. Other plugins may now use this thread.
This is done for the media plugins. The mplayer video plugin only starts the mplayer thread and register it to the plugin interface as VIDEO_PLAYER. The video item asks the plugin interface to give him the VIDEO_PLAYER.