RapidWeaver comes with some really awesome page styles built right in, however some of the page styles are sort of "one trick wonders".
For instance the Contact page does a good job at creating a contact page, but doesnt allow you to insert content above or below the form. Well, EC makes it possible.
Whether its the Photo Album page, Movie Album page, or the File Sharing page adding content above or below the content areas was either impossible or very difficult. With EC, you can place content anywhere you like!
The Standout theme includes 4 General EC areas and 3 "theme specific" EC areas.
Have you heard of Loghounds PlusKit? It has an amazing feature called @import that can pull any page into another page by adding one bit of special "@import" code.
When combined with the power of EC you become a professional web developer overnight ... well, at least thats what people will think.