My parents purchased it before they went on the Disney cruise. We have never purchased it before a regular Disney trip.
The trip insurance my parents got protected them if someone got sick before the trip and the whole family was unable to go. They would get reimbursed for their flights and for the price of the cruise. Also, it paid for emergency treatment during the cruise (helicopter rescue, etc).
I find that I have unforeseen problems so infrequently that I'm better off paying the costs associated with them myself, out of my own savings, rather than paying premiums for travel insurance for each trip.
A cruise is different, since there are some possible costs that would exceed my savings account.