What color is it? The darker colors are more forgiving. I don't have a Mickey Bag but I do have a number of classic
Coach bags and I have found them to be very tough and resilient. Does your bag have a stiff, shiny finish or is it soft and flexible? The stiff, shiny finish tends to scratch more easily.
If I had a Mickey bag I would want to use it at Disney World, but then I wouldn't want to be worried about it all the time, either. I learned how to use and enjoy my bags - but also protect them - when I was taking crowded public transportation and walking to work in all kinds of weather.
I always protect my bags by spraying them with Apple Rain Garde. I also take the precaution of carrying, a lightweight plastic bag with me. If the weather suddenly turns bad I can pull out the plastic bag and shove my beloved Coach bag into it. I carry a large, plastic drawstring shopping bag from the Nike Outlet. I lay it out flat, smooth it out, and then fold it up into a small square. It doesn't take up much room and it has come in handy.
I don't know how big or heavy your Mickey bag is, but walking around Disney World all day can get very tiring and sometimes the classic leather bags can get a bit heavy. I would want to carry a light-weight, hands-free comfortable bag and keep the contents to a minimum during an all-day outing. I only have 2 signature fabric Coach bags but they are perfect for an all-day activity and they are crossbody so I can wear them hands-free. I also have a small, leather crossbody swingpack with a built-in wallet that I use when I only want to carry a few things with me.
You might consider saving the Mickey Saddle Bag for going out to dinner or similar activities while you are in the Orlando area and use a light-weight and less precious bag for trips to the theme parks.
Ask yourself these questions "How sad will I feel if I don't carry Mickey at Disney World?" and "How bad will I feel if my Mickey Bag gets stained or scratched?" The answers to those questions should help you to make up your mind; have fun!