By staying onsite, you are guaranteed admission to A PARK, not necessarily the one of your choice. For example, if MK is full, they may direct you to Epcot.
Someone asked this question around Christmas and they said if you cannot get in to go to guest relations and they will give you a refund. I don't think MGM closes too often and usually a closed park reopens later in the day as people leave.
Are you flying in? I'd be more concerned about a delayed flight.