In today's economy, everyone is trying to cut costs and save money. A Save the Date card can help you get an idea of who is planning on coming to your event or wedding.
You should treat a save the date card the same as you would an invitation -- you need to send them to everyone you intend to invite. Remember, people will be excited when they receive your news and they will talk, email, tweet and facebook to everyone that might also know you "Did you get the Save the Date for Dave and Karen's Wedding!"
After you've sent the save the date cards, you will probably get some informal feedback as to who will and will not be able to attend your wedding so might be able to use that information to help you budget.
As the save the date cards are an informal notice, I would still send everyone on your save the date list a wedding invitation as that is when you ask invited guests to RSVP and you will then have your final head count for your wedding or event.