I felt the same way last year for our trip to Disney. The weather called for rain every day of our vacation! My Aunt (used to live in Kissimmee) told me that the weather always calls for rain in the summer.
As gschmerl said, they are short showers. It cools everything off - I love it.
Don't worry and don't let it ruin your plans. They don't last long and when they do, the parks empty. Go have a snack while it rains, then go enjoy the park!
If it rains take advantage of it and go to universal studios. Most people when the rain comes they head for the hills. But don't run take advantage of it. The universal side has mostly indoor attractions and you shouldn't get to wet going from one attraction to another.
I don't remember the rain cooling things off (or not for long). It thunderstormed everyday but not for long. Can't use the pool when it is an electrical storm. We had an evening of steady drizzle and just put on ponchos or a lightweight jacket with a hood.
The past 3 days we have been getting the storms around 2 and then lasting for the rest of the night. Some times they are heavy and then other light and don't forget about the lightning. No body has been struck yet but it caused about a 100 acre fire near UCF and a house fire.