I was going to summarize this article, but I think it's better if you just read it yourself.
Most current veterinarians wish we could vaccinate less, but we were hindered. First, none of the products on the market were labeled for 3 year cycles. This meant that we exposed ourselves to tremendous liability if we used the vaccine against label directions. Second, while we strongly suspected the vaccine was good for more than 1 year, there was no scientific evidence supporting the increased time between vaccinations. And lastly, the old guys in town were telling everybody that if we didn't vaccinate every year the dogs would die a horrible death and it would be all our fault.
But now we have a choice. Major veterinary organizations have changed their recommendations to better reflect the current information.
Now, this is not an excuse to NEVER vaccinate your dog, or to skip the annual exam. Dogs die from Canine distemper every day. I've seen it and it's not pretty. Plus, since your dog or cat can't talk to you, an annual physical is important. And I would go to any doctor who just gave me my shots and turned me out the door.