I really don't understand the point of these notices. In order to send a save-the-date notificiation, you must already know when the wedding is, and if you know when the wedding is going to be, you almost certainly know where it's going to be held, especially if you're the type inclined to send out such notices. Furthermore, you know who you're going to be inviting because you wouldn't send a save-the-date notice to somebody who you aren't inviting. So, if you know all the critical details, why not just send out the invitations?
My hypothesis is that some couple actually had a legitimite need to send out such notices, such as an impending wedding date without a confirmed location. But some bachelorette received received one of these notices and decided her wedding had to have at least everything that her disorganized friend's did. And at some point, the stationer or wedding planner added save-the-dates to the checklist, and we'll be forever subject to such silliness until there is a cure to bridal irrationality syndrome.
The state is that great fiction by which everyone tries to live at the expense of everyone else. - Frederic Bastiat