For the consumers, they are curious to see what it will look like, but I think it could be a good strategy for Sony. Imagine if Sony managed to fit all of the tech into a real slim and stylish casing, while the next Xbox created a stylish casing, but the console being bigger. If Microsoft saw that the PS4 would be a lot smaller while on par, they might rethink their design. Of course, with holiday 2013 being the launch window, that gives Sony more than enough time to show the console design.
In addition, I did read someones comment on an NBC article on why Sony didn't show the console design. They argument was that a lot of people care about putting customize stickers on they consoles, so not showing the design hurts these companies that makes these stickers.