Yup, saving with layers takes up lot more room than a flattened image.
Most people save both ways b/c it's only WITH the layers that you can go back to your page and change things on it i.e. a spelling error, a funny drop shadow, etc. If you save only the flattened, then you will have to rebuild the layer from scratch in the future if you need to go back.
I don't usually save my layered files indefinitely, but at least for 3-6 months after I initially create the page. I know lots of people DO save layered and flattened indefinitely, but I just don't have the space to do that.
HTH!
P.S. I like your screenname.
