IMHO... they are a courtesy that I feel is necessary for a DW unless you want to send out your invites way in advance. As other posters have said - people need time to save money, take time off of work, obtain passports, etc. It's just a nice little extra to say that "We would like for you to be there with us and want to give you enough time to plan for it". You don't have to do anything expensive ... DIY ones are great!
We sent ours out in October 2008 for our August 2009 wedding and most people appreciated the advance notice and loved that we spend the time to send them something special; even if they couldn't attend. Our invites went out in March 2009 to everyone (40 people) even though we know that only 17 people could attend and had already booked. And again, I received a lot of comments from people appreciating the courtesy of including them whether or not they were attending.