Dec. 2002, we were supposed to go to
MVMCP---and the night we had pre-bought our tickets for, there was a torrential downpour! I am from Florida- so I know about rain, but this was not rain, it was impossible to stay dry - even with a rain parka and umbrella; quite a bit of flooding too- given that the drains couldn't keep up with the amount of water falling from the sky.
While they didn't cancel MVMCP, they readily admitted that the parades, outdoor shows, and fireworks wouldn't be shown- so we were able to get a refund for our tickets (unused). We had to mail them into Disney, and my credit card was credited for the entire amount.
Disney is used to a little rain - and most shows will still go on (even the fireworks); but in the event of torrential downpours, they will give you a refund.
As the other posters have said though- if you are going during a busy time (the closer to Christmas, the busier it will be), or on a Friday - then you might want to buy your tickets in advance just in case it sells out --Jill