My personal opinion is that kennel cough is like a human cold. Most of the time not that big a deal. Definitely a pain though. Like a human cold occasionally turns into bronchitis or pneumonia, the same can happen with a dog. My dogs have had kennel cough on a variety of occasions and it usually is just a few days of coughing and then they're back to normal. I used to vaccinate for it when I worked at a dog daycare that required it (since I took my dogs to work). I found that one of my dogs who has a bad immune system still caught it all the time and the other never did. In fact, I'm pretty sure he caught it off my clothes after I taught a puppy class one time (found out later that one of the puppies came down with it the next day, but my dog was not at class that night). Now that I've stopped vaccinating, my low immune system boy still gets it once in a while, the other two still never get it. I personally haven't had any of my boys catch it at a dock jumping event, but it definitely is a possibility. That said, the fun my boys and I have at the events far outweighs the risk of catching a cold in my opinion.
That said, I would hope that anyone who knows their dog has kennel cough would keep their dog home that day!
Just my two cents...