We ALWAYS pack a sweatshirt for each person in our carryons.
The plane can be chilly.
And every once in a while we get a really chilly, wet, windy day at WDW (yes, even in August. We're teachers and only go over the summer.). We got one in 2008 as tropical storm Faye approached. It was cold enough that, after getting soaked on Kali, we were wet and miserable-- went back, changed, grabbed our sweathirts, and hopped to Epcot.
Most of the time, they spend in the closet of our resort. But having one along is a whole lot more convenient than having to puchase one.
Oh, and one more generic packing hint: we always place one outfit apiece into someone else's luggage. So I'll have one of my son's, he'll have one my his sister's, she'll have one of my husband's, and so on.
In the unlikely event that one bag gets temporarily misplaced, at least it buys you a day.