I would think that Disney would encourage you to buy a ticket for MORE days than you were going to use in 1 visit......that way it's a win-win for them! Either you 1)*waste* the ticket by never using it, and Disney gets to keep your money, or 2) you come back for another visit, and Disney gets even MORE of your money!
I know that sounded rather cynical....don't get me wrong, I LOVE Disney.....but it seems to me they've never passed up a chance to make a buck!