On my last trip, Feb 2004, it poured out almost every day. You know what? We LOVED IT!!
It made the parks empty. We walked on to pretty much every ride over and over again. It was great. We barely got wet because we had umbrellas and we were mostly inside buildings.
If it rains, the parks clear out. And the rain usually does not last long. Maybe an hour.
There was only one day when the rain was unbearable, and that was because we were at MGM and there was a lot more space between rides.