The following is not at all limited or tailored to Disney's insurance, but I'm pretty sure it works like other travel insurance that I've had occasion to learn a little about in my professional life.
Whether you want to buy the insurance depends on how risk-averse you are. But you have to be quite risk-averse (or have unusual knowledge of an increased likelihood of a covered event that might cause you the trip to be cancelled) to make it worthwhile, because on a strictly financial basis, your expected return on travel insurance is significantly less than putting the same amount of money in a slot machine. Companies that sell travel insurance generally make a lot of money on it -- not as high a percentage as they make on extended warranties, but still quite high.
If you take vacations annually or more frequently, you'll do better over time to put the money you might otherwise spend on travel insurance aside in a rainy day fund.
OTOH, if you have saved up a long time for one special trip, and losing the money from that trip will mean you can't take another one for a long time, you might just be risk-averse enough to make the peace of mind that comes from having insurance worth the cost. Oddly, though, people in that position are less likely to think they can afford the travel insurance!
