Two Sundays ago we went to MK in the rain. It was a steady rain but not pouring. We wore our ponchos and stayed at MK for about 2.5 hours. We rode all the indoor rides and just walked right on. All rides were working. After awhile we just got too wet and decided to go back to the room and take a warm bath. But after lunch we went to AK and it was empty. The rain had stopped, but the park stayed empty. We had a safari car to ourselves.
On another drizzly day, we went to DTD so we could use our 7th day of our pass on our last day at WDW. But if I had to do it again, I would have gone to a park. There's lots of indoors things you can do to stay out of the rain most of the time. And once you see how crowded the parks are in the sunshine, it makes it all worthwhile.