Our first year, my father in law was ill.... not "deathly ill", just in perennial poor health. (As it turns out, he passed away about a month after our return.)
And, as teachers, we only go in August, prime hurricane season. In 2008, Tropical Storm Faye wasn't on the radar when we left NY, but arrived in Orlando the day we were due to fly home. I faxed our travel agent, asked what we should do, and left for the Magic Kingdom, happy that we had a plan.
With all the other costs involved in a Disney trip, it simply makes sense for me to invest a little bit into "what if?"