If you have no medical insurance, or if you don't have medical insurance that covers you away from home, a travel medical policy is a good idea.
Most medical policies cover you away from home, but some don't do it very well.
The policy sold by Disney is not a particularly good policy, and some of the aftermarket policies are less expensive and better coverage.
I don't get it becuase my medical insurance does a good job anywhere in the world, and the majority of the trip is refundable or changeable until right before I travel.
I have found that many organizations are understanding of TRUE emergencies and will do all they can to help you if something goes really wrong.
Mostly it comes down to personal choice and your risk tolerance.
If someone has a medical condition that might be a problem, it might be a good idea to get a policy, but make sure it covers pre-exisiting conditions.