I have gone to WDW in September, which is the rainy season, many times.
I hate rain, especially on vacation, but when it rains at WDW, I put on rain gear and keep going. Most of my favorite attractions are indoors anyway, and the rain does tend to decrease the park crowds.
One thing to think about - if you look at the forcast and the rain is expected to continue steadily all day, don't just take a plastic poncho and call it done. Continuous rain means you need long pants and some means of warmth; even in September I was wearing long pants to keep the A/C in the buildings from freezing me solid!