This is what happens....
So I have had experience in dealing with Dinner Package Guests as well as rain shows....
Essentially, you never actually 'pay' for a seat at Fantasmic! as part of the dining package, so therefore you are not losing on something you never bought... if that makes sense, and you also actually recieve a better deal on your food and drink, then if you were to just walk in and dine...so essentially regardless of if you want to see the show or not, you are better of with the dinner package as you recieve cheaper food...
so that being said when a show is cancelled, or a rain show performed there is nothing really on offer to be replaced, you could go to guest relations and depending on your 'anger level' they may arrange for you to attend the next showing....
My advice, go to the later show... it rarely fills up (besides thanksgiving, and christmas/new years week), you can literally turn up 10 mins before show time and take a seat, as opposed to the 30-45mins reccommended for dinner Package, and the seats will be just as good if not better... xD