We brought the cheap-o $1 Store ponchos. They are VERY thin but they did not get too hot. We used them 3 of the 6 days we were at parks. It rained for at least 15 minutes almost every day, and on some days it then stopped, but one day it rained lightly from about 5 pm until 9:30 or so (we were at EPCOT and watched Illuminations with relatively low crowds b/c of the drizzle). A couple of times, we just stayed in a shop for a fw minutes and the rian cleared up; if it lasted more than 15 minutes or so, we busted out the ponchos.
Having lived in Ft. Lauderdale, it is true that it rains almost every day in the summer, usually in the afternoon, but the storms are generally short. I advocate bringing the cheap-o ponchos. Not only are they not as heavy to wear, but they are not too heavy to carry around the park until the rain starts.