I think something overlooked is if you don't bring it and have to get it there, you also have to get it home. that might mean disney priced luggage and another unexpected bag fee on top of disney priced clothing.. and disney does not have a large supplies of long leg or cold weather jackets so you might be having a nice afternoon at a nearby store instead of in disney. I don't think they sell any beanies or gloves. stores at springs might and there is a couple countries in epcot that might as well but overall it's hard to find the cold weather clothing so your looking at layers and layers of not so cold clothing when you buy it.
i'd rather pack 1, maybe 2 days of other weather gear in an extra suitcase and bring it with us and doing extra laundry then to have to deal with finding it, the cost of it and bringing it home.
edit: I saw other weather because if the 10 day forecast says mostly cold, I'm not skipping the warm clothes and if it's saying mostly warm, I'm not skipping some cold weather clothes..
Lockers in the parks will be your friend for layers and not having to carry it around or run back to the resort.