One thing I can think of specific resorts or rooms may be available with the bounce back discount at the time a guest is staying at WDW when they may not be available when a special promotion comes out that covers the dates a person wants to visit. Another is that the good rates may not come out as they normally do. I'm guessing that an early reservation will lock in the prices at that time so there is also a possibility for a rack rate increase before the next promotion.