I am definitely in the minority here, but I will be in Disney 8/18-8/26 and I most definitely will bring a light jacket. Just enough to take the chill off in a restaurant. I agree with the others that you certainly do not need it outside, but if you tend to be a cold person (like me) while everyone else is comfy, then bring one.
My dh and kids like the room cooler than me, so I just put my jacket on. I will use my jacket when we are on the airplane. (If we drove, which we don't, but if we did I would bring my down comforter - yikes I know. My mom says my blood is cold.)
The one time we were there in late May and under normal circumstances I would never wear a jacket, but I was having a reaction to the sun and the skin on my arms couldn't tolerate the sun, so I just put my jacket on. I looked crazy but I could go outside and push a stroller.
Bring it, if you don't use it, then so what. It's not like it takes up a ton of space.