Usually buy it each summer trip in Aug as it is hurricane season. This year thru Disney, in past thru online company.
I have nit used disneys but I will say that years back a trip to Cabo that was several thousands of dollars was canceled due to illness and we got back ALL our money.
Some people like to chance it.....others don't....I think disney says it'll charge 200 if you cancel within 5 days of trip (plus any other fess...you'd have to read it)
So, its a personal decision....if you can affird to LOSE the $$ in the event of a cancellation, forgo it, if it would stress you out to lose the money (which also covers air, etc) buy it, it is relatively cheap compared to lost $$$.
Also, be VERY careful to look at policies, pre existing injuries/illness (to you AND others) is Excluded....some policies look back a certain number of days.....Basically, you can "add" that in if you want...I usually do when it is not thru disney....and most say that you have to add the Ins within 5 days of final payment........
ASk questions to be sure you get what you want....I have used insure my trip dot com and they were great in my experience with them/
Good Luck, hope this helped a bit........