I get this question all of the time. Do I really need Save the Date Cards? It's just another expense. My response to this question usually goes something like: Either you would like to do all you can to assist the people you've invited to your wedding in attending or you should take them off the guest list if it doesn't matter.
Look at it this way. You have spent the last 12 or so months of your life consumed with the planning of every intricate detail of this day, don't you want those people you invite to show? Everyone has things going on in their lives: birthdays, vacations, anniversaries, graduations, etc. The more lead time they have in knowing of your event, the more leeway they have in keeping the date clear of other engagements.
Don't overdo it
When to send them
Keep in mind that all you want your guests to do at this point is save the date. Don't give away all the plans you've made. Also, don't expect a response. Your responses will come on that pretty little postage paid response card you've had printed.
If you are planning a destination wedding, send your Save the Date cards at least six months in advance. Follow the card with a detailed mailing informing guests of information which will be crucial to their travel.